docs: compare cold grid and translated pan rendering alternatives

harness/experiments/grid-cache-20260925/COLD-PAN.md

@@ -0,0 +1,153 @@
+# Cold construction and pan alternatives
+
+Follow-up to [the stationary-camera cache](README.md), for issue 44. Application
+source remains `336720559bcec511949caa8a42bee8a55c3822da`, with evidence checkpoint
+`2ca84676ee4dc30846208881a27d90a901482085`. These standalone experiments change no
+application code, dependencies, event handling or release. Native interaction
+remains paused; no result below establishes native FPS.
+
+## Question and method
+
+The existing cache removes repeated construction at identical camera inputs but
+misses during pan/zoom. Can simpler geometry or renderer-level translation remove
+that cost without changing the grid's appearance? Use the same unoptimized Iced
+0.14 WGPU dependencies and Windows environment recorded in `context.json`.
+Compile the new drivers with the same `rustc`/extern/linker arguments described
+in README, replacing the driver filename and output executable. No desktop window
+is created and no input is sent. Run `cold_grid_probe` with argument `20`; phase
+drivers take no arguments.
+
+The cold driver compares original circles, one reusable origin circle transformed
+for each fill, one path per column, and direct 1.15-unit squares. Every candidate
+also constructs one origin-circle template; this small common overhead is retained.
+Three warmups precede 20 changing-pan samples. Construction ends before geometry
+destruction, rendering and readback. Percentiles use sorted index `ceil((n-1)*p)`.
+Fixed candidate ordering and uncontrolled OS load limit small timing comparisons.
+
+## Cold construction results
+
+`cold_grid_probe.rs` produces `grid-cold-01.log`. Median milliseconds:
+
+| Canvas, zoom | Original | Reused circle | Column batch | Square |
+| --- | ---: | ---: | ---: | ---: |
+| 1900x660, 1.0 | 61.0564 | 55.3975 | 54.3594 | 3.5527 |
+| 1900x660, 0.5 | 215.8919 | 162.3073 | 158.5968 | 7.5699 |
+| 1440x900, 1.0 | 60.1683 | 43.3860 | 60.8198 | 2.0340 |
+| 1440x900, 0.5 | 165.2049 | 133.3420 | 143.1489 | 6.8178 |
+
+Both circle alternatives exactly match original pixels in all 12 sampled cases
+(four integer canvas/zoom pairs, three pan offsets, scale 1). This does not prove
+equivalence at fractional zoom, other DPI or other camera inputs. Squares match
+eight of those cases but differ at the third pan offset in each canvas/zoom pair:
+4,266–13,184 changed pixels, maximum channel difference 7. Their different shape
+and area are an explicit appearance change, not a pixel-preserving optimization.
+
+Reusing circles and column batches leave substantial cold cost; the experiment
+does not justify adding them to production as the solution to low FPS. The square
+alternative is much faster in this scope, but is not adopted without evaluating
+its appearance and the actual interaction result. The previous all-grid single
+path experiment remains rejected for its much higher construction cost.
+
+## Source explanation and independent review
+
+A fresh read-only critic checked the probes and actual local Iced source. Each
+`Path::circle` allocates/builds a path and converts its arc into quadratic segments.
+Each WGPU `Frame::fill` allocates a boxed geometry builder and invokes Lyon fill
+tessellation. Same-color fills already share a solid mesh buffer: this is not
+one GPU draw call per dot. Frame translation transforms paths before tessellation,
+so path reuse alone does not reuse triangles.
+
+Renderer-level `with_translation` can reuse cached triangles. Stock Canvas cannot
+attach a separate transform to each returned geometry; application adoption would
+need a small background widget or wrapper, with layout, clipping, lifecycle and
+interaction checks. Scaling the grid to handle zoom is unsuitable because the
+dot radius must remain fixed in screen units.
+
+The critic also identified two important controls. Original opaque background
+geometry must not silently become screenshot-clear color: MSAA/alpha composition
+can change quantized pixels. Also, `iced_wgpu::triangle::Storage::prepare` rewrites
+cached mesh data when its transform changes. Construction/command-recording time
+therefore excludes real preparation/upload costs. The later probe measures a
+rendering/readback path separately; neither timer represents native presentation.
+
+Primary sources: Iced 0.14 `iced_graphics/src/geometry/path{,/builder}.rs`,
+`iced_wgpu/src/{geometry,triangle}.rs`, `iced_core/src/renderer.rs`, and
+`iced_widget/src/canvas.rs`. The reviewer changed no files and did not execute the
+probes; execution and interpretation remain the parent's responsibility.
+
+## Renderer-translated phase cache
+
+`phase_grid_probe_v1.rs` / `grid-phase-01.log` cache zero-phase circles once for
+each size/spacing pair, translate by `pan.rem_euclid(step)`, and externally clip
+to the original viewport. The original nonnegative row/column indices and fixed
+screen-space radius are retained. Cache clip bounds include the radius margins.
+This first prototype uses screenshot clear color instead of opaque background
+geometry; its widespread one-level pixel differences are not accepted as parity.
+Its compiler emitted one duplicate Renderer-import warning, retained in the
+original driver. Later drivers remove that import and compile with `-D warnings`.
+
+`phase_grid_opaque_probe.rs` / `grid-phase-opaque-01.log` add an oversized opaque
+background to the same cached geometry as the circles. Its bounds extend from
+`(-step,-step)` to `(width+2*step,height+2*step)`, covering the viewport at every
+positive remainder without adding extra dot rows or columns. This directly
+compares against the unchanged legacy painter. A proposed separate-background
+control was not executed; no result is claimed for it.
+
+Of 75 comparisons (five size/zoom pairs, five offsets, three DPI scales), 73 are
+byte-exact. All 60 cases at integer canvas sizes and spacing 14 or 24 are exact.
+At 301.5x211.25, zoom 0.73, pan (-13.25,9.75), scale 1 changes 12 pixels by at
+most one channel level; scale 2 changes one pixel by at most six levels. These
+are failures of exact parity and are preserved, not hidden behind a tolerance.
+
+The targeted `phase_grid_diagnostic.rs` / `grid-phase-diagnostic-01.log` locates
+all 12 scale-1 differences at the rightmost physical column x=301. The scale-2
+single-pixel difference does not recur. That diagnostic skips earlier large
+cases and their rendering/timing sequence: it does not exonerate the earlier
+failure or establish that every residual difference is at an edge.
+
+The replay driver's first compilation failed with E0689 because the new control's
+`Vector::new(-13.25,9.75)` left its numeric type ambiguous before `rem_euclid`.
+An explicit `Vector<f32>` annotation corrects the experiment harness. No probe
+executed during that failed attempt; no application source changed.
+
+`phase_grid_replay.rs` preserves the full opaque probe's rendering/timing sequence
+and adds coordinate diagnostics without extra renders until that sequence ends.
+`grid-phase-replay-01.log` reproduces both failures: the 12 right-edge pixels and
+one **interior** pixel at physical (149,263), where legacy RGBA is
+`[240,243,246,255]` and cached RGBA is `[246,248,251,255]`. The latter cannot be
+dismissed as viewport-edge rounding. Afterwards, all 12 same-renderer repeat
+controls (three repetitions of old/old and cached/cached at two scales) are
+byte-exact; the control cache builds once. This finite control does not establish
+universal determinism, but supports investigating differing geometry coverage
+instead of treating the reproduced pixel as random readback noise.
+
+Cold construction plus command recording takes 68.42–173.72ms median for the four
+large viewports; cached translation takes 0.0008–0.0011ms. Each per-size/spacing
+cache builds exactly once through all pan comparisons and measurements. Zoom
+and resize through a single retained cache are not tested by these drivers.
+The labels containing `submit` mean renderer command recording, not GPU submission.
+
+Two selected cases include construction, recording, backend preparation and GPU
+readback in a separate timer (two warmups, eight samples, sorted-index median):
+
+| Canvas, zoom | Original p50 ms | Phase cache p50 ms |
+| --- | ---: | ---: |
+| 1900x660, 1.0 | 259.9265 | 212.6536 |
+| 1440x900, 0.5 | 337.2560 | 170.3780 |
+
+These support a benefit within the headless path, but retain significant renderer
+and readback cost and variable outliers. They are not native frame times. Sample
+size, sequential ordering and uncontrolled machine load prohibit a precise
+general speedup claim. All driver/log hashes and execution mapping are in
+`cold-pan-context.json`.
+
+The phase candidate removes the measured pan tessellation cost, but does not
+meet exact visual preservation. Do not adopt it as an invisible optimization.
+The small circle rewrites leave the main cost and squares change appearance.
+Further work should investigate the reproduced interior coverage difference or
+compare optimized dependency/build profiles before adding a custom widget.
+Any adopted rendering candidate must then verify a real mounted scene at a
+nonzero widget origin, foreground ordering, viewport clipping, pan/zoom/resize
+cache lifecycle and unchanged event behavior. Native interaction/FPS verification
+remains a separate required gate. No application implementation is changed by
+these experiments.

harness/experiments/grid-cache-20260925/README.md

@@ -139,6 +139,11 @@ remains deferred by the user. No published release has been replaced.
 
 ## Environment and reproduction
 
+The subsequent [cold-construction and pan experiments](COLD-PAN.md) compare
+additional approaches, including a renderer-translated cache. They preserve
+their own pixel differences, timing limits and adoption gates; they do not
+change the production implementation verified above.
+
 Windows 10.0.26200, Rust 1.98.1, x86_64-pc-windows-gnullvm, development profile
 with debug information disabled. The exposed CPU is Intel Core i7-10870H with
 4 cores/threads; Windows reports VMware SVGA 3D, driver 9.17.11.4. The headless

harness/experiments/grid-cache-20260925/cold-pan-context.json

@@ -0,0 +1,51 @@
+{
+  "application_source": "336720559bcec511949caa8a42bee8a55c3822da",
+  "base_evidence": "2ca84676ee4dc30846208881a27d90a901482085",
+  "production_changes": false,
+  "environment": "Same unoptimized Windows Iced/Tokio artifacts as context.json",
+  "logs": [
+    {
+      "driver": "cold_grid_probe.rs",
+      "exit_code": 0,
+      "log": "grid-cold-01.log"
+    },
+    {
+      "driver": "phase_grid_probe_v1.rs",
+      "exit_code": 0,
+      "compile_warning": "Unused duplicate Renderer trait import; retained in original driver",
+      "log": "grid-phase-01.log"
+    },
+    {
+      "driver": "phase_grid_opaque_probe.rs",
+      "exit_code": 0,
+      "compile_warnings_denied": true,
+      "log": "grid-phase-opaque-01.log"
+    },
+    {
+      "driver": "phase_grid_diagnostic.rs",
+      "exit_code": 0,
+      "compile_warnings_denied": true,
+      "log": "grid-phase-diagnostic-01.log"
+    },
+    {
+      "driver": "phase_grid_replay.rs",
+      "log": "grid-phase-replay-01.log",
+      "exit_code": 0,
+      "compile_warnings_denied": true,
+      "initial_compile": "E0689 ambiguous Vector scalar before rem_euclid, fixed with Vector<f32>; no execution on failed attempt"
+    }
+  ],
+  "sha256": {
+    "cold_grid_probe.rs": "a4cc6543229ae4901d54061823a3b05ec6fe3b988f245df12898e013dbb93323",
+    "phase_grid_probe_v1.rs": "c4293e8bb022fae5bffe67002a6f7c867549f2fa8a865b107f39070655bc57fc",
+    "phase_grid_opaque_probe.rs": "43ac5be462ca31f9d1c49554291e08b60bac9de38face7138eaa850e4dcc0ab9",
+    "phase_grid_diagnostic.rs": "8de3c980ac65eef4fcb32b1b121d14aca35b93c704c5b19a440ad9dc7042248c",
+    "grid-cold-01.log": "ca5cac0be409bafcea2c154d42998cbd622c9e898892574bcd5a2df0c1a70886",
+    "grid-phase-01.log": "e0b2e2a3e24d642e1b6c2b1b418f24c952399758159de051f7c1ba7efbb39c4a",
+    "grid-phase-opaque-01.log": "2716cbeb589f4b23a48575680022231c0b94b56ba9c06ec1adbf9e3e6d07f7cd",
+    "grid-phase-diagnostic-01.log": "1dbb0f745169b88c56a21683dba6797a3dbbe753e8dd742bc9ecc8ca24788886",
+    "phase_grid_replay.rs": "96e62ba5d08f7e26d9705d3f90e3785151597b526293439345b59ed842de4971",
+    "grid-phase-replay-01.log": "040034be1b8eb3d4fc010bbe34c5ded47a953b8295bcf2f9440507d1a149da1f"
+  },
+  "native_fps": "unverified"
+}

harness/experiments/grid-cache-20260925/cold_grid_probe.rs

@@ -0,0 +1,142 @@
+//! Experimental cold-grid candidates; no window or input automation.
+use iced::advanced::{
+    graphics::geometry::Renderer as _,
+    renderer::{Headless, Renderer as _},
+};
+use iced::{
+    Color, Point, Rectangle, Renderer, Size, Vector,
+    widget::canvas::{Frame, Geometry, Path},
+};
+use std::{hint::black_box, time::Instant};
+const BG: Color = Color::from_rgb8(246, 248, 251);
+const DOT: Color = Color::from_rgb(0.86, 0.89, 0.91);
+
+#[derive(Debug, Clone, Copy)]
+enum Candidate {
+    Original,
+    Template,
+    Column,
+    Square,
+}
+fn draw(r: &Renderer, size: Size, zoom: f32, pan: Vector, candidate: Candidate) -> Geometry {
+    let mut f = Frame::new(r, size);
+    f.fill_rectangle(Point::ORIGIN, size, BG);
+    let step = (24. * zoom).max(14.);
+    let ox = pan.x.rem_euclid(step);
+    let oy = pan.y.rem_euclid(step);
+    let circle = Path::circle(Point::ORIGIN, 0.65);
+    for x in 0..(size.width / step) as i32 + 1 {
+        if matches!(candidate, Candidate::Column) {
+            let p = Path::new(|p| {
+                for y in 0..(size.height / step) as i32 + 1 {
+                    p.circle(Point::new(ox + x as f32 * step, oy + y as f32 * step), 0.65);
+                    p.close();
+                }
+            });
+            f.fill(&p, DOT);
+            continue;
+        }
+        for y in 0..(size.height / step) as i32 + 1 {
+            let center = Point::new(ox + x as f32 * step, oy + y as f32 * step);
+            match candidate {
+                Candidate::Original => f.fill(&Path::circle(center, 0.65), DOT),
+                Candidate::Template => f.with_save(|f| {
+                    f.translate(Vector::new(center.x, center.y));
+                    f.fill(&circle, DOT);
+                }),
+                Candidate::Square => f.fill_rectangle(
+                    center - Vector::new(0.575, 0.575),
+                    Size::new(1.15, 1.15),
+                    DOT,
+                ),
+                Candidate::Column => unreachable!(),
+            }
+        }
+    }
+    f.into_geometry()
+}
+fn pixels(r: &mut Renderer, size: Size, g: Geometry) -> Vec<u8> {
+    r.reset(Rectangle::with_size(size));
+    r.draw_geometry(g);
+    Headless::screenshot(r, Size::new(size.width as u32, size.height as u32), 1., BG)
+}
+fn main() {
+    let n = std::env::args()
+        .nth(1)
+        .map(|s| s.parse::<usize>().unwrap())
+        .unwrap_or(20)
+        .max(1);
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .unwrap();
+    let mut r = rt
+        .block_on(<Renderer as Headless>::new(
+            iced::Font::DEFAULT,
+            16.into(),
+            None,
+        ))
+        .unwrap();
+    println!(
+        "renderer={} n={n}; construction only, unoptimized linked dependencies; NOT native FPS",
+        r.name()
+    );
+    for (size, zoom) in [
+        (Size::new(1900., 660.), 1.),
+        (Size::new(1900., 660.), 0.5),
+        (Size::new(1440., 900.), 1.),
+        (Size::new(1440., 900.), 0.5),
+    ] {
+        println!("size={}x{} zoom={zoom}", size.width, size.height);
+        for candidate in [
+            Candidate::Original,
+            Candidate::Template,
+            Candidate::Column,
+            Candidate::Square,
+        ] {
+            for pan in [
+                Vector::new(-13.25, 9.75),
+                Vector::new(0., 0.),
+                Vector::new(6.125, -8.5),
+            ] {
+                let base = draw(&r, size, zoom, pan, Candidate::Original);
+                let base = pixels(&mut r, size, base);
+                let test = draw(&r, size, zoom, pan, candidate);
+                let test = pixels(&mut r, size, test);
+                let changed = base
+                    .chunks_exact(4)
+                    .zip(test.chunks_exact(4))
+                    .filter(|(a, b)| a != b)
+                    .count();
+                let max = base
+                    .iter()
+                    .zip(&test)
+                    .map(|(a, b)| a.abs_diff(*b))
+                    .max()
+                    .unwrap_or(0);
+                println!(
+                    "{candidate:?} pan={},{} changed_pixels={changed} max_channel_delta={max}",
+                    pan.x, pan.y
+                );
+            }
+            let mut ms = Vec::new();
+            for i in 0..n + 3 {
+                let pan = Vector::new(-13.25 + i as f32 * 0.25, 9.75);
+                let now = Instant::now();
+                let g = black_box(draw(&r, size, zoom, pan, candidate));
+                let elapsed = now.elapsed().as_secs_f64() * 1000.;
+                drop(g);
+                if i >= 3 {
+                    ms.push(elapsed);
+                }
+            }
+            ms.sort_by(f64::total_cmp);
+            println!(
+                "{candidate:?} changing_pan n={n} p50_ms={:.4} p95_ms={:.4} max_ms={:.4}",
+                ms[((n - 1) as f64 * 0.5).ceil() as usize],
+                ms[((n - 1) as f64 * 0.95).ceil() as usize],
+                ms[n - 1]
+            );
+        }
+    }
+}

harness/experiments/grid-cache-20260925/grid-cold-01.log

@@ -0,0 +1,69 @@
+renderer=wgpu n=20; construction only, unoptimized linked dependencies; NOT native FPS
+size=1900x660 zoom=1
+Original pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Original pan=0,0 changed_pixels=0 max_channel_delta=0
+Original pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Original changing_pan n=20 p50_ms=61.0564 p95_ms=111.2137 max_ms=111.2137
+Template pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Template pan=0,0 changed_pixels=0 max_channel_delta=0
+Template pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Template changing_pan n=20 p50_ms=55.3975 p95_ms=73.8790 max_ms=73.8790
+Column pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Column pan=0,0 changed_pixels=0 max_channel_delta=0
+Column pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Column changing_pan n=20 p50_ms=54.3594 p95_ms=69.6607 max_ms=69.6607
+Square pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Square pan=0,0 changed_pixels=0 max_channel_delta=0
+Square pan=6.125,-8.5 changed_pixels=4266 max_channel_delta=7
+Square changing_pan n=20 p50_ms=3.5527 p95_ms=5.6911 max_ms=5.6911
+size=1900x660 zoom=0.5
+Original pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Original pan=0,0 changed_pixels=0 max_channel_delta=0
+Original pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Original changing_pan n=20 p50_ms=215.8919 p95_ms=253.9666 max_ms=253.9666
+Template pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Template pan=0,0 changed_pixels=0 max_channel_delta=0
+Template pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Template changing_pan n=20 p50_ms=162.3073 p95_ms=212.2833 max_ms=212.2833
+Column pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Column pan=0,0 changed_pixels=0 max_channel_delta=0
+Column pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Column changing_pan n=20 p50_ms=158.5968 p95_ms=449.6066 max_ms=449.6066
+Square pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Square pan=0,0 changed_pixels=0 max_channel_delta=0
+Square pan=6.125,-8.5 changed_pixels=12784 max_channel_delta=7
+Square changing_pan n=20 p50_ms=7.5699 p95_ms=13.3416 max_ms=13.3416
+size=1440x900 zoom=1
+Original pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Original pan=0,0 changed_pixels=0 max_channel_delta=0
+Original pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Original changing_pan n=20 p50_ms=60.1683 p95_ms=88.4967 max_ms=88.4967
+Template pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Template pan=0,0 changed_pixels=0 max_channel_delta=0
+Template pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Template changing_pan n=20 p50_ms=43.3860 p95_ms=53.1866 max_ms=53.1866
+Column pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Column pan=0,0 changed_pixels=0 max_channel_delta=0
+Column pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Column changing_pan n=20 p50_ms=60.8198 p95_ms=80.8932 max_ms=80.8932
+Square pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Square pan=0,0 changed_pixels=0 max_channel_delta=0
+Square pan=6.125,-8.5 changed_pixels=4440 max_channel_delta=7
+Square changing_pan n=20 p50_ms=2.0340 p95_ms=4.0620 max_ms=4.0620
+size=1440x900 zoom=0.5
+Original pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Original pan=0,0 changed_pixels=0 max_channel_delta=0
+Original pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Original changing_pan n=20 p50_ms=165.2049 p95_ms=203.5331 max_ms=203.5331
+Template pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Template pan=0,0 changed_pixels=0 max_channel_delta=0
+Template pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Template changing_pan n=20 p50_ms=133.3420 p95_ms=143.9790 max_ms=143.9790
+Column pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Column pan=0,0 changed_pixels=0 max_channel_delta=0
+Column pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+Column changing_pan n=20 p50_ms=143.1489 p95_ms=155.3680 max_ms=155.3680
+Square pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+Square pan=0,0 changed_pixels=0 max_channel_delta=0
+Square pan=6.125,-8.5 changed_pixels=13184 max_channel_delta=7
+Square changing_pan n=20 p50_ms=6.8178 p95_ms=10.8775 max_ms=10.8775

harness/experiments/grid-cache-20260925/grid-phase-01.log

@@ -0,0 +1,96 @@
+renderer=wgpu; phase-cache prototype; NOT native FPS
+size=1900x660 zoom=1 step=24
+scale=1 pan=0,0 changed_pixels=0 max_channel_delta=0
+scale=1 pan=-13.25,9.75 changed_pixels=2212 max_channel_delta=1
+scale=1 pan=6.125,-8.5 changed_pixels=2133 max_channel_delta=1
+scale=1 pan=23.875,23.75 changed_pixels=2133 max_channel_delta=1
+scale=1 pan=-0.125,0.125 changed_pixels=4424 max_channel_delta=1
+scale=1.25 pan=0,0 changed_pixels=8745 max_channel_delta=1
+scale=1.25 pan=-13.25,9.75 changed_pixels=2212 max_channel_delta=1
+scale=1.25 pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+scale=1.25 pan=23.875,23.75 changed_pixels=4266 max_channel_delta=1
+scale=1.25 pan=-0.125,0.125 changed_pixels=6557 max_channel_delta=1
+scale=2 pan=0,0 changed_pixels=0 max_channel_delta=0
+scale=2 pan=-13.25,9.75 changed_pixels=8848 max_channel_delta=1
+scale=2 pan=6.125,-8.5 changed_pixels=4266 max_channel_delta=1
+scale=2 pan=23.875,23.75 changed_pixels=10665 max_channel_delta=1
+scale=2 pan=-0.125,0.125 changed_pixels=6478 max_channel_delta=1
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harness/experiments/grid-cache-20260925/grid-phase-diagnostic-01.log

@@ -0,0 +1,32 @@
+renderer=wgpu; phase-cache prototype; NOT native FPS
+size=301.5x211.25 zoom=0.73 step=17.52
+scale=1 pan=0,0 changed_pixels=0 max_channel_delta=0
+difference_xy=301,26 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,27 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,62 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,80 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,97 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,115 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,132 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,150 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,167 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,184 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,185 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
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harness/experiments/grid-cache-20260925/grid-phase-opaque-01.log

@@ -0,0 +1,100 @@
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harness/experiments/grid-cache-20260925/grid-phase-replay-01.log

@@ -0,0 +1,127 @@
+renderer=wgpu; phase-cache prototype; NOT native FPS
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+scale=2 pan=0,0 changed_pixels=0 max_channel_delta=0
+scale=2 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+scale=2 pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+scale=2 pan=13.875,13.75 changed_pixels=0 max_channel_delta=0
+scale=2 pan=-0.125,0.125 changed_pixels=0 max_channel_delta=0
+cached=false construct_and_submit changing_pan n=20 p50_ms=209.4728 p95_ms=329.0977 max_ms=329.0977
+cached=true construct_and_submit changing_pan n=20 p50_ms=0.0008 p95_ms=0.0009 max_ms=0.0009
+cache_builds=1
+cached=false construct_submit_screenshot changing_pan n=8 p50_ms=384.7680 min_ms=347.4879 max_ms=420.2894
+cached=true construct_submit_screenshot changing_pan n=8 p50_ms=164.1611 min_ms=150.1276 max_ms=186.4567
+size=301.5x211.25 zoom=0.73 step=17.52
+scale=1 pan=0,0 changed_pixels=0 max_channel_delta=0
+scale=1 pan=-13.25,9.75 changed_pixels=12 max_channel_delta=1
+difference_xy=301,26 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,27 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,62 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,80 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,97 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,115 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,132 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,150 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,167 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,184 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,185 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+difference_xy=301,202 expected=[240, 243, 247, 255] actual=[240, 243, 246, 255]
+scale=1 pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+scale=1 pan=17.395,17.27 changed_pixels=0 max_channel_delta=0
+scale=1 pan=-0.125,0.125 changed_pixels=0 max_channel_delta=0
+scale=1.25 pan=0,0 changed_pixels=0 max_channel_delta=0
+scale=1.25 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+scale=1.25 pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+scale=1.25 pan=17.395,17.27 changed_pixels=0 max_channel_delta=0
+scale=1.25 pan=-0.125,0.125 changed_pixels=0 max_channel_delta=0
+scale=2 pan=0,0 changed_pixels=0 max_channel_delta=0
+scale=2 pan=-13.25,9.75 changed_pixels=1 max_channel_delta=6
+difference_xy=149,263 expected=[240, 243, 246, 255] actual=[246, 248, 251, 255]
+scale=2 pan=6.125,-8.5 changed_pixels=0 max_channel_delta=0
+scale=2 pan=17.395,17.27 changed_pixels=0 max_channel_delta=0
+scale=2 pan=-0.125,0.125 changed_pixels=0 max_channel_delta=0
+cached=false construct_and_submit changing_pan n=20 p50_ms=5.5680 p95_ms=7.0634 max_ms=7.0634
+cached=true construct_and_submit changing_pan n=20 p50_ms=0.0007 p95_ms=0.0008 max_ms=0.0008
+cache_builds=1
+original_sequence_complete; beginning same-renderer repeat controls
+comparison=old/old repetition=1 size=301.5x211.25 scale=1 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=cached/cached repetition=1 size=301.5x211.25 scale=1 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=old/old repetition=1 size=301.5x211.25 scale=2 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=cached/cached repetition=1 size=301.5x211.25 scale=2 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=old/old repetition=2 size=301.5x211.25 scale=1 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=cached/cached repetition=2 size=301.5x211.25 scale=1 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=old/old repetition=2 size=301.5x211.25 scale=2 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=cached/cached repetition=2 size=301.5x211.25 scale=2 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=old/old repetition=3 size=301.5x211.25 scale=1 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=cached/cached repetition=3 size=301.5x211.25 scale=1 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=old/old repetition=3 size=301.5x211.25 scale=2 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+comparison=cached/cached repetition=3 size=301.5x211.25 scale=2 pan=-13.25,9.75 changed_pixels=0 max_channel_delta=0
+control_cache_builds=1

harness/experiments/grid-cache-20260925/phase_grid_diagnostic.rs

@@ -0,0 +1,209 @@
+//! Renderer-level phase translation experiment, not application code.
+use iced::advanced::{Renderer as _, graphics::geometry::Renderer as _, renderer::Headless};
+use iced::{
+    Color, Point, Rectangle, Renderer, Size, Vector,
+    widget::canvas::{Cache, Frame, Geometry, Path},
+};
+use std::{cell::Cell, hint::black_box, time::Instant};
+const BG: Color = Color::from_rgb8(246, 248, 251);
+const DOT: Color = Color::from_rgb(0.86, 0.89, 0.91);
+fn dots(f: &mut Frame, size: Size, step: f32, phase: Vector) {
+    for x in 0..(size.width / step) as i32 + 1 {
+        for y in 0..(size.height / step) as i32 + 1 {
+            f.fill(
+                &Path::circle(
+                    Point::new(phase.x + x as f32 * step, phase.y + y as f32 * step),
+                    0.65,
+                ),
+                DOT,
+            );
+        }
+    }
+}
+fn old(r: &Renderer, size: Size, step: f32, phase: Vector) -> Geometry {
+    let mut f = Frame::new(r, size);
+    f.fill_rectangle(Point::ORIGIN, size, BG);
+    dots(&mut f, size, step, phase);
+    f.into_geometry()
+}
+fn phase(r: &Renderer, cache: &Cache, size: Size, step: f32, builds: &Cell<usize>) -> Geometry {
+    cache.draw_with_bounds(
+        r,
+        Rectangle {
+            x: -step,
+            y: -step,
+            width: size.width + 3. * step,
+            height: size.height + 3. * step,
+        },
+        |f| {
+            builds.set(builds.get() + 1);
+            f.fill_rectangle(
+                Point::new(-step, -step),
+                Size::new(size.width + 3. * step, size.height + 3. * step),
+                BG,
+            );
+            dots(f, size, step, Vector::ZERO);
+        },
+    )
+}
+fn submit(r: &mut Renderer, size: Size, g: Geometry, pan: Vector) {
+    r.with_layer(Rectangle::with_size(size), |r| {
+        r.with_translation(pan, |r| r.draw_geometry(g))
+    });
+}
+fn shot(r: &mut Renderer, size: Size, scale: f32) -> Vec<u8> {
+    Headless::screenshot(
+        r,
+        Size::new(
+            (size.width * scale).ceil() as u32,
+            (size.height * scale).ceil() as u32,
+        ),
+        scale,
+        BG,
+    )
+}
+fn main() {
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .unwrap();
+    let mut r = rt
+        .block_on(<Renderer as Headless>::new(
+            iced::Font::DEFAULT,
+            16.into(),
+            None,
+        ))
+        .unwrap();
+    println!(
+        "renderer={}; phase-cache prototype; NOT native FPS",
+        r.name()
+    );
+    for (size, zoom) in [
+        (Size::new(1900., 660.), 1.),
+        (Size::new(1900., 660.), 0.5),
+        (Size::new(1440., 900.), 1.),
+        (Size::new(1440., 900.), 0.5),
+        (Size::new(301.5, 211.25), 0.73),
+    ] {
+        if size.width != 301.5 {
+            continue;
+        }
+        let step = (24f32 * zoom).max(14.);
+        let cache = Cache::new();
+        let builds = Cell::new(0);
+        println!(
+            "size={}x{} zoom={zoom} step={step}",
+            size.width, size.height
+        );
+        for scale in [1., 1.25, 2.] {
+            for pan in [
+                Vector::ZERO,
+                Vector::new(-13.25, 9.75),
+                Vector::new(6.125, -8.5),
+                Vector::new(step - 0.125, step - 0.25),
+                Vector::new(-0.125, 0.125),
+            ] {
+                let offset = Vector::new(pan.x.rem_euclid(step), pan.y.rem_euclid(step));
+                r.reset(Rectangle::with_size(size));
+                let g = old(&r, size, step, offset);
+                r.draw_geometry(g);
+                let expected = shot(&mut r, size, scale);
+                r.reset(Rectangle::with_size(size));
+                let g = phase(&r, &cache, size, step, &builds);
+                submit(&mut r, size, g, offset);
+                let actual = shot(&mut r, size, scale);
+                assert_eq!(expected.len(), actual.len());
+                for (index, (a, b)) in expected
+                    .chunks_exact(4)
+                    .zip(actual.chunks_exact(4))
+                    .enumerate()
+                    .filter(|(_, (a, b))| a != b)
+                    .take(20)
+                {
+                    println!(
+                        "difference_xy={},{} expected={a:?} actual={b:?}",
+                        index % (size.width * scale).ceil() as usize,
+                        index / (size.width * scale).ceil() as usize
+                    );
+                }
+                let changed = expected
+                    .chunks_exact(4)
+                    .zip(actual.chunks_exact(4))
+                    .filter(|(a, b)| a != b)
+                    .count();
+                let max = expected
+                    .iter()
+                    .zip(&actual)
+                    .map(|(a, b)| a.abs_diff(*b))
+                    .max()
+                    .unwrap_or(0);
+                println!(
+                    "scale={scale} pan={},{} changed_pixels={changed} max_channel_delta={max}",
+                    pan.x, pan.y
+                );
+            }
+        }
+        assert_eq!(builds.get(), 1);
+        for cached in [false, true] {
+            let mut ms = Vec::new();
+            for i in 0..23 {
+                r.reset(Rectangle::with_size(size));
+                let offset = Vector::new((i as f32 * 0.25).rem_euclid(step), 9.75);
+                let now = Instant::now();
+                let g = if cached {
+                    phase(&r, &cache, size, step, &builds)
+                } else {
+                    old(&r, size, step, offset)
+                };
+                if cached {
+                    submit(&mut r, size, g, offset)
+                } else {
+                    r.draw_geometry(g)
+                };
+                let elapsed = black_box(now.elapsed().as_secs_f64() * 1000.);
+                if i >= 3 {
+                    ms.push(elapsed);
+                }
+            }
+            ms.sort_by(f64::total_cmp);
+            println!(
+                "cached={cached} construct_and_submit changing_pan n=20 p50_ms={:.4} p95_ms={:.4} max_ms={:.4}",
+                ms[10], ms[19], ms[19]
+            );
+        }
+        println!("cache_builds={}", builds.get());
+        assert_eq!(builds.get(), 1);
+        // Includes backend prepare/upload and readback; still not native presentation.
+        if size.width == 1900. && zoom == 1. || size.width == 1440. && zoom == 0.5 {
+            for cached in [false, true] {
+                let mut ms = Vec::new();
+                for i in 0..10 {
+                    r.reset(Rectangle::with_size(size));
+                    let offset = Vector::new((i as f32 * 0.25).rem_euclid(step), 9.75);
+                    let now = Instant::now();
+                    let g = if cached {
+                        phase(&r, &cache, size, step, &builds)
+                    } else {
+                        old(&r, size, step, offset)
+                    };
+                    if cached {
+                        submit(&mut r, size, g, offset);
+                    } else {
+                        r.draw_geometry(g);
+                    }
+                    let pixels = black_box(shot(&mut r, size, 1.));
+                    let elapsed = now.elapsed().as_secs_f64() * 1000.;
+                    drop(pixels);
+                    if i >= 2 {
+                        ms.push(elapsed);
+                    }
+                }
+                ms.sort_by(f64::total_cmp);
+                println!(
+                    "cached={cached} construct_submit_screenshot changing_pan n=8 p50_ms={:.4} min_ms={:.4} max_ms={:.4}",
+                    ms[4], ms[0], ms[7]
+                );
+            }
+        }
+    }
+}

harness/experiments/grid-cache-20260925/phase_grid_opaque_probe.rs

@@ -0,0 +1,193 @@
+//! Renderer-level phase translation experiment, not application code.
+use iced::advanced::{Renderer as _, graphics::geometry::Renderer as _, renderer::Headless};
+use iced::{
+    Color, Point, Rectangle, Renderer, Size, Vector,
+    widget::canvas::{Cache, Frame, Geometry, Path},
+};
+use std::{cell::Cell, hint::black_box, time::Instant};
+const BG: Color = Color::from_rgb8(246, 248, 251);
+const DOT: Color = Color::from_rgb(0.86, 0.89, 0.91);
+fn dots(f: &mut Frame, size: Size, step: f32, phase: Vector) {
+    for x in 0..(size.width / step) as i32 + 1 {
+        for y in 0..(size.height / step) as i32 + 1 {
+            f.fill(
+                &Path::circle(
+                    Point::new(phase.x + x as f32 * step, phase.y + y as f32 * step),
+                    0.65,
+                ),
+                DOT,
+            );
+        }
+    }
+}
+fn old(r: &Renderer, size: Size, step: f32, phase: Vector) -> Geometry {
+    let mut f = Frame::new(r, size);
+    f.fill_rectangle(Point::ORIGIN, size, BG);
+    dots(&mut f, size, step, phase);
+    f.into_geometry()
+}
+fn phase(r: &Renderer, cache: &Cache, size: Size, step: f32, builds: &Cell<usize>) -> Geometry {
+    cache.draw_with_bounds(
+        r,
+        Rectangle {
+            x: -step,
+            y: -step,
+            width: size.width + 3. * step,
+            height: size.height + 3. * step,
+        },
+        |f| {
+            builds.set(builds.get() + 1);
+            f.fill_rectangle(
+                Point::new(-step, -step),
+                Size::new(size.width + 3. * step, size.height + 3. * step),
+                BG,
+            );
+            dots(f, size, step, Vector::ZERO);
+        },
+    )
+}
+fn submit(r: &mut Renderer, size: Size, g: Geometry, pan: Vector) {
+    r.with_layer(Rectangle::with_size(size), |r| {
+        r.with_translation(pan, |r| r.draw_geometry(g))
+    });
+}
+fn shot(r: &mut Renderer, size: Size, scale: f32) -> Vec<u8> {
+    Headless::screenshot(
+        r,
+        Size::new(
+            (size.width * scale).ceil() as u32,
+            (size.height * scale).ceil() as u32,
+        ),
+        scale,
+        BG,
+    )
+}
+fn main() {
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .unwrap();
+    let mut r = rt
+        .block_on(<Renderer as Headless>::new(
+            iced::Font::DEFAULT,
+            16.into(),
+            None,
+        ))
+        .unwrap();
+    println!(
+        "renderer={}; phase-cache prototype; NOT native FPS",
+        r.name()
+    );
+    for (size, zoom) in [
+        (Size::new(1900., 660.), 1.),
+        (Size::new(1900., 660.), 0.5),
+        (Size::new(1440., 900.), 1.),
+        (Size::new(1440., 900.), 0.5),
+        (Size::new(301.5, 211.25), 0.73),
+    ] {
+        let step = (24f32 * zoom).max(14.);
+        let cache = Cache::new();
+        let builds = Cell::new(0);
+        println!(
+            "size={}x{} zoom={zoom} step={step}",
+            size.width, size.height
+        );
+        for scale in [1., 1.25, 2.] {
+            for pan in [
+                Vector::ZERO,
+                Vector::new(-13.25, 9.75),
+                Vector::new(6.125, -8.5),
+                Vector::new(step - 0.125, step - 0.25),
+                Vector::new(-0.125, 0.125),
+            ] {
+                let offset = Vector::new(pan.x.rem_euclid(step), pan.y.rem_euclid(step));
+                r.reset(Rectangle::with_size(size));
+                let g = old(&r, size, step, offset);
+                r.draw_geometry(g);
+                let expected = shot(&mut r, size, scale);
+                r.reset(Rectangle::with_size(size));
+                let g = phase(&r, &cache, size, step, &builds);
+                submit(&mut r, size, g, offset);
+                let actual = shot(&mut r, size, scale);
+                assert_eq!(expected.len(), actual.len());
+                let changed = expected
+                    .chunks_exact(4)
+                    .zip(actual.chunks_exact(4))
+                    .filter(|(a, b)| a != b)
+                    .count();
+                let max = expected
+                    .iter()
+                    .zip(&actual)
+                    .map(|(a, b)| a.abs_diff(*b))
+                    .max()
+                    .unwrap_or(0);
+                println!(
+                    "scale={scale} pan={},{} changed_pixels={changed} max_channel_delta={max}",
+                    pan.x, pan.y
+                );
+            }
+        }
+        assert_eq!(builds.get(), 1);
+        for cached in [false, true] {
+            let mut ms = Vec::new();
+            for i in 0..23 {
+                r.reset(Rectangle::with_size(size));
+                let offset = Vector::new((i as f32 * 0.25).rem_euclid(step), 9.75);
+                let now = Instant::now();
+                let g = if cached {
+                    phase(&r, &cache, size, step, &builds)
+                } else {
+                    old(&r, size, step, offset)
+                };
+                if cached {
+                    submit(&mut r, size, g, offset)
+                } else {
+                    r.draw_geometry(g)
+                };
+                let elapsed = black_box(now.elapsed().as_secs_f64() * 1000.);
+                if i >= 3 {
+                    ms.push(elapsed);
+                }
+            }
+            ms.sort_by(f64::total_cmp);
+            println!(
+                "cached={cached} construct_and_submit changing_pan n=20 p50_ms={:.4} p95_ms={:.4} max_ms={:.4}",
+                ms[10], ms[19], ms[19]
+            );
+        }
+        println!("cache_builds={}", builds.get());
+        assert_eq!(builds.get(), 1);
+        // Includes backend prepare/upload and readback; still not native presentation.
+        if size.width == 1900. && zoom == 1. || size.width == 1440. && zoom == 0.5 {
+            for cached in [false, true] {
+                let mut ms = Vec::new();
+                for i in 0..10 {
+                    r.reset(Rectangle::with_size(size));
+                    let offset = Vector::new((i as f32 * 0.25).rem_euclid(step), 9.75);
+                    let now = Instant::now();
+                    let g = if cached {
+                        phase(&r, &cache, size, step, &builds)
+                    } else {
+                        old(&r, size, step, offset)
+                    };
+                    if cached {
+                        submit(&mut r, size, g, offset);
+                    } else {
+                        r.draw_geometry(g);
+                    }
+                    let pixels = black_box(shot(&mut r, size, 1.));
+                    let elapsed = now.elapsed().as_secs_f64() * 1000.;
+                    drop(pixels);
+                    if i >= 2 {
+                        ms.push(elapsed);
+                    }
+                }
+                ms.sort_by(f64::total_cmp);
+                println!(
+                    "cached={cached} construct_submit_screenshot changing_pan n=8 p50_ms={:.4} min_ms={:.4} max_ms={:.4}",
+                    ms[4], ms[0], ms[7]
+                );
+            }
+        }
+    }
+}

harness/experiments/grid-cache-20260925/phase_grid_probe_v1.rs

@@ -0,0 +1,160 @@
+//! Renderer-level phase translation experiment, not application code.
+use iced::advanced::{
+    Renderer as _,
+    graphics::geometry::Renderer as _,
+    renderer::{Headless, Renderer as _},
+};
+use iced::{
+    Color, Point, Rectangle, Renderer, Size, Vector,
+    widget::canvas::{Cache, Frame, Geometry, Path},
+};
+use std::{cell::Cell, hint::black_box, time::Instant};
+const BG: Color = Color::from_rgb8(246, 248, 251);
+const DOT: Color = Color::from_rgb(0.86, 0.89, 0.91);
+fn dots(f: &mut Frame, size: Size, step: f32, phase: Vector) {
+    for x in 0..(size.width / step) as i32 + 1 {
+        for y in 0..(size.height / step) as i32 + 1 {
+            f.fill(
+                &Path::circle(
+                    Point::new(phase.x + x as f32 * step, phase.y + y as f32 * step),
+                    0.65,
+                ),
+                DOT,
+            );
+        }
+    }
+}
+fn old(r: &Renderer, size: Size, step: f32, phase: Vector) -> Geometry {
+    let mut f = Frame::new(r, size);
+    f.fill_rectangle(Point::ORIGIN, size, BG);
+    dots(&mut f, size, step, phase);
+    f.into_geometry()
+}
+fn phase(r: &Renderer, cache: &Cache, size: Size, step: f32, builds: &Cell<usize>) -> Geometry {
+    cache.draw_with_bounds(
+        r,
+        Rectangle {
+            x: -1.,
+            y: -1.,
+            width: size.width + step + 2.,
+            height: size.height + step + 2.,
+        },
+        |f| {
+            builds.set(builds.get() + 1);
+            dots(f, size, step, Vector::ZERO);
+        },
+    )
+}
+fn submit(r: &mut Renderer, size: Size, g: Geometry, pan: Vector) {
+    r.with_layer(Rectangle::with_size(size), |r| {
+        r.with_translation(pan, |r| r.draw_geometry(g))
+    });
+}
+fn shot(r: &mut Renderer, size: Size, scale: f32) -> Vec<u8> {
+    Headless::screenshot(
+        r,
+        Size::new(
+            (size.width * scale).ceil() as u32,
+            (size.height * scale).ceil() as u32,
+        ),
+        scale,
+        BG,
+    )
+}
+fn main() {
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .unwrap();
+    let mut r = rt
+        .block_on(<Renderer as Headless>::new(
+            iced::Font::DEFAULT,
+            16.into(),
+            None,
+        ))
+        .unwrap();
+    println!(
+        "renderer={}; phase-cache prototype; NOT native FPS",
+        r.name()
+    );
+    for (size, zoom) in [
+        (Size::new(1900., 660.), 1.),
+        (Size::new(1900., 660.), 0.5),
+        (Size::new(1440., 900.), 1.),
+        (Size::new(1440., 900.), 0.5),
+        (Size::new(301.5, 211.25), 0.73),
+    ] {
+        let step = (24f32 * zoom).max(14.);
+        let cache = Cache::new();
+        let builds = Cell::new(0);
+        println!(
+            "size={}x{} zoom={zoom} step={step}",
+            size.width, size.height
+        );
+        for scale in [1., 1.25, 2.] {
+            for pan in [
+                Vector::ZERO,
+                Vector::new(-13.25, 9.75),
+                Vector::new(6.125, -8.5),
+                Vector::new(step - 0.125, step - 0.25),
+                Vector::new(-0.125, 0.125),
+            ] {
+                let offset = Vector::new(pan.x.rem_euclid(step), pan.y.rem_euclid(step));
+                r.reset(Rectangle::with_size(size));
+                let g = old(&r, size, step, offset);
+                r.draw_geometry(g);
+                let expected = shot(&mut r, size, scale);
+                r.reset(Rectangle::with_size(size));
+                let g = phase(&r, &cache, size, step, &builds);
+                submit(&mut r, size, g, offset);
+                let actual = shot(&mut r, size, scale);
+                assert_eq!(expected.len(), actual.len());
+                let changed = expected
+                    .chunks_exact(4)
+                    .zip(actual.chunks_exact(4))
+                    .filter(|(a, b)| a != b)
+                    .count();
+                let max = expected
+                    .iter()
+                    .zip(&actual)
+                    .map(|(a, b)| a.abs_diff(*b))
+                    .max()
+                    .unwrap_or(0);
+                println!(
+                    "scale={scale} pan={},{} changed_pixels={changed} max_channel_delta={max}",
+                    pan.x, pan.y
+                );
+            }
+        }
+        assert_eq!(builds.get(), 1);
+        for cached in [false, true] {
+            let mut ms = Vec::new();
+            for i in 0..23 {
+                r.reset(Rectangle::with_size(size));
+                let offset = Vector::new((i as f32 * 0.25).rem_euclid(step), 9.75);
+                let now = Instant::now();
+                let g = if cached {
+                    phase(&r, &cache, size, step, &builds)
+                } else {
+                    old(&r, size, step, offset)
+                };
+                if cached {
+                    submit(&mut r, size, g, offset)
+                } else {
+                    r.draw_geometry(g)
+                };
+                let elapsed = black_box(now.elapsed().as_secs_f64() * 1000.);
+                if i >= 3 {
+                    ms.push(elapsed);
+                }
+            }
+            ms.sort_by(f64::total_cmp);
+            println!(
+                "cached={cached} construct_and_submit changing_pan n=20 p50_ms={:.4} p95_ms={:.4} max_ms={:.4}",
+                ms[10], ms[19], ms[19]
+            );
+        }
+        println!("cache_builds={}", builds.get());
+        assert_eq!(builds.get(), 1);
+    }
+}

harness/experiments/grid-cache-20260925/phase_grid_replay.rs

@@ -0,0 +1,285 @@
+//! Full-sequence opaque phase replay with diagnostics and repeat controls; not application code.
+use iced::advanced::{Renderer as _, graphics::geometry::Renderer as _, renderer::Headless};
+use iced::{
+    Color, Point, Rectangle, Renderer, Size, Vector,
+    widget::canvas::{Cache, Frame, Geometry, Path},
+};
+use std::{cell::Cell, hint::black_box, time::Instant};
+const BG: Color = Color::from_rgb8(246, 248, 251);
+const DOT: Color = Color::from_rgb(0.86, 0.89, 0.91);
+fn dots(f: &mut Frame, size: Size, step: f32, phase: Vector) {
+    for x in 0..(size.width / step) as i32 + 1 {
+        for y in 0..(size.height / step) as i32 + 1 {
+            f.fill(
+                &Path::circle(
+                    Point::new(phase.x + x as f32 * step, phase.y + y as f32 * step),
+                    0.65,
+                ),
+                DOT,
+            );
+        }
+    }
+}
+fn old(r: &Renderer, size: Size, step: f32, phase: Vector) -> Geometry {
+    let mut f = Frame::new(r, size);
+    f.fill_rectangle(Point::ORIGIN, size, BG);
+    dots(&mut f, size, step, phase);
+    f.into_geometry()
+}
+fn phase(r: &Renderer, cache: &Cache, size: Size, step: f32, builds: &Cell<usize>) -> Geometry {
+    cache.draw_with_bounds(
+        r,
+        Rectangle {
+            x: -step,
+            y: -step,
+            width: size.width + 3. * step,
+            height: size.height + 3. * step,
+        },
+        |f| {
+            builds.set(builds.get() + 1);
+            f.fill_rectangle(
+                Point::new(-step, -step),
+                Size::new(size.width + 3. * step, size.height + 3. * step),
+                BG,
+            );
+            dots(f, size, step, Vector::ZERO);
+        },
+    )
+}
+fn submit(r: &mut Renderer, size: Size, g: Geometry, pan: Vector) {
+    r.with_layer(Rectangle::with_size(size), |r| {
+        r.with_translation(pan, |r| r.draw_geometry(g))
+    });
+}
+fn shot(r: &mut Renderer, size: Size, scale: f32) -> Vec<u8> {
+    Headless::screenshot(
+        r,
+        Size::new(
+            (size.width * scale).ceil() as u32,
+            (size.height * scale).ceil() as u32,
+        ),
+        scale,
+        BG,
+    )
+}
+// Diagnostics only: this performs no rendering and does not mutate the renderer.
+fn log_coordinates(expected: &[u8], actual: &[u8], size: Size, scale: f32) {
+    assert_eq!(expected.len(), actual.len());
+    let physical_width = (size.width * scale).ceil() as usize;
+    for (index, (a, b)) in expected
+        .as_chunks::<4>()
+        .0
+        .iter()
+        .zip(actual.as_chunks::<4>().0)
+        .enumerate()
+        .filter(|(_, (a, b))| a != b)
+        .take(20)
+    {
+        println!(
+            "difference_xy={},{} expected={a:?} actual={b:?}",
+            index % physical_width,
+            index / physical_width
+        );
+    }
+}
+
+fn report_control(label: &str, expected: &[u8], actual: &[u8], size: Size, scale: f32) {
+    assert_eq!(expected.len(), actual.len());
+    let changed = expected
+        .as_chunks::<4>()
+        .0
+        .iter()
+        .zip(actual.as_chunks::<4>().0)
+        .filter(|(a, b)| a != b)
+        .count();
+    let max = expected
+        .iter()
+        .zip(actual)
+        .map(|(a, b)| a.abs_diff(*b))
+        .max()
+        .unwrap_or(0);
+    println!(
+        "comparison={label} size={}x{} scale={scale} pan=-13.25,9.75 changed_pixels={changed} max_channel_delta={max}",
+        size.width, size.height
+    );
+    log_coordinates(expected, actual, size, scale);
+}
+fn main() {
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .unwrap();
+    let mut r = rt
+        .block_on(<Renderer as Headless>::new(
+            iced::Font::DEFAULT,
+            16.into(),
+            None,
+        ))
+        .unwrap();
+    println!(
+        "renderer={}; phase-cache prototype; NOT native FPS",
+        r.name()
+    );
+    for (size, zoom) in [
+        (Size::new(1900., 660.), 1.),
+        (Size::new(1900., 660.), 0.5),
+        (Size::new(1440., 900.), 1.),
+        (Size::new(1440., 900.), 0.5),
+        (Size::new(301.5, 211.25), 0.73),
+    ] {
+        let step = (24f32 * zoom).max(14.);
+        let cache = Cache::new();
+        let builds = Cell::new(0);
+        println!(
+            "size={}x{} zoom={zoom} step={step}",
+            size.width, size.height
+        );
+        for scale in [1., 1.25, 2.] {
+            for pan in [
+                Vector::ZERO,
+                Vector::new(-13.25, 9.75),
+                Vector::new(6.125, -8.5),
+                Vector::new(step - 0.125, step - 0.25),
+                Vector::new(-0.125, 0.125),
+            ] {
+                let offset = Vector::new(pan.x.rem_euclid(step), pan.y.rem_euclid(step));
+                r.reset(Rectangle::with_size(size));
+                let g = old(&r, size, step, offset);
+                r.draw_geometry(g);
+                let expected = shot(&mut r, size, scale);
+                r.reset(Rectangle::with_size(size));
+                let g = phase(&r, &cache, size, step, &builds);
+                submit(&mut r, size, g, offset);
+                let actual = shot(&mut r, size, scale);
+                assert_eq!(expected.len(), actual.len());
+                let changed = expected
+                    .chunks_exact(4)
+                    .zip(actual.chunks_exact(4))
+                    .filter(|(a, b)| a != b)
+                    .count();
+                let max = expected
+                    .iter()
+                    .zip(&actual)
+                    .map(|(a, b)| a.abs_diff(*b))
+                    .max()
+                    .unwrap_or(0);
+                println!(
+                    "scale={scale} pan={},{} changed_pixels={changed} max_channel_delta={max}",
+                    pan.x, pan.y
+                );
+                // Keep the original render/timing sequence; print diagnostics only.
+                log_coordinates(&expected, &actual, size, scale);
+            }
+        }
+        assert_eq!(builds.get(), 1);
+        for cached in [false, true] {
+            let mut ms = Vec::new();
+            for i in 0..23 {
+                r.reset(Rectangle::with_size(size));
+                let offset = Vector::new((i as f32 * 0.25).rem_euclid(step), 9.75);
+                let now = Instant::now();
+                let g = if cached {
+                    phase(&r, &cache, size, step, &builds)
+                } else {
+                    old(&r, size, step, offset)
+                };
+                if cached {
+                    submit(&mut r, size, g, offset)
+                } else {
+                    r.draw_geometry(g)
+                };
+                let elapsed = black_box(now.elapsed().as_secs_f64() * 1000.);
+                if i >= 3 {
+                    ms.push(elapsed);
+                }
+            }
+            ms.sort_by(f64::total_cmp);
+            println!(
+                "cached={cached} construct_and_submit changing_pan n=20 p50_ms={:.4} p95_ms={:.4} max_ms={:.4}",
+                ms[10], ms[19], ms[19]
+            );
+        }
+        println!("cache_builds={}", builds.get());
+        assert_eq!(builds.get(), 1);
+        // Includes backend prepare/upload and readback; still not native presentation.
+        if size.width == 1900. && zoom == 1. || size.width == 1440. && zoom == 0.5 {
+            for cached in [false, true] {
+                let mut ms = Vec::new();
+                for i in 0..10 {
+                    r.reset(Rectangle::with_size(size));
+                    let offset = Vector::new((i as f32 * 0.25).rem_euclid(step), 9.75);
+                    let now = Instant::now();
+                    let g = if cached {
+                        phase(&r, &cache, size, step, &builds)
+                    } else {
+                        old(&r, size, step, offset)
+                    };
+                    if cached {
+                        submit(&mut r, size, g, offset);
+                    } else {
+                        r.draw_geometry(g);
+                    }
+                    let pixels = black_box(shot(&mut r, size, 1.));
+                    let elapsed = now.elapsed().as_secs_f64() * 1000.;
+                    drop(pixels);
+                    if i >= 2 {
+                        ms.push(elapsed);
+                    }
+                }
+                ms.sort_by(f64::total_cmp);
+                println!(
+                    "cached={cached} construct_submit_screenshot changing_pan n=8 p50_ms={:.4} min_ms={:.4} max_ms={:.4}",
+                    ms[4], ms[0], ms[7]
+                );
+            }
+        }
+    }
+    // All original cases, timings and screenshots have completed above.
+    // Reuse the same renderer for controls. This cache is newly constructed,
+    // then retained across all control repetitions; original per-case caches
+    // have already gone out of scope, exactly as in the opaque probe.
+    println!("original_sequence_complete; beginning same-renderer repeat controls");
+    let size = Size::new(301.5, 211.25);
+    let step = (24f32 * 0.73).max(14.);
+    let pan: Vector<f32> = Vector::new(-13.25, 9.75);
+    let offset = Vector::new(pan.x.rem_euclid(step), pan.y.rem_euclid(step));
+    let cache = Cache::new();
+    let builds = Cell::new(0);
+    for repetition in 1..=3 {
+        for scale in [1., 2.] {
+            r.reset(Rectangle::with_size(size));
+            let g = old(&r, size, step, offset);
+            r.draw_geometry(g);
+            let first = shot(&mut r, size, scale);
+            r.reset(Rectangle::with_size(size));
+            let g = old(&r, size, step, offset);
+            r.draw_geometry(g);
+            let second = shot(&mut r, size, scale);
+            report_control(
+                &format!("old/old repetition={repetition}"),
+                &first,
+                &second,
+                size,
+                scale,
+            );
+
+            r.reset(Rectangle::with_size(size));
+            let g = phase(&r, &cache, size, step, &builds);
+            submit(&mut r, size, g, offset);
+            let first = shot(&mut r, size, scale);
+            r.reset(Rectangle::with_size(size));
+            let g = phase(&r, &cache, size, step, &builds);
+            submit(&mut r, size, g, offset);
+            let second = shot(&mut r, size, scale);
+            report_control(
+                &format!("cached/cached repetition={repetition}"),
+                &first,
+                &second,
+                size,
+                scale,
+            );
+        }
+    }
+    println!("control_cache_builds={}", builds.get());
+    assert_eq!(builds.get(), 1);
+}