Experiment with unwired shaped label field coverage

Preserve winning-buffer cluster geometry and exact scalar mappings. Distinguish truncated source from unresolved glyph coverage, keep ellipsis unmapped, and add ten precondition-asserted field evaluators. Source-only self-review and diff check; not formatted, compiled or executed. No production module wiring or navigation policy adoption.

crates/oa-desktop/src/behavior/scenario_label.rs

@@ -18,6 +18,9 @@
 //! It measures layout advances, not raster ink; overhang/clip/activation-border
 //! acceptance still requires the real WGPU evaluator. The old 32-grapheme/225px
 //! renderer is deliberately untouched for a later baseline comparison.
+//! Field geometry is a second unwired experiment: exact retained source spans
+//! and winning-buffer cluster boxes, with unresolved glyph coverage distinct
+//! from truncation. It does not adopt a production navigation policy.
 use iced::advanced::{
     graphics::text as graphics_text,
     text::{Alignment, LineHeight, Shaping},
@@ -94,9 +97,121 @@ pub(super) struct Label {
     style: Style,
     measured_session: RenderSession,
     measured_zoom: f32,
+    // Snapshot of the final displayed Cache entry, after alignment. These are
+    // cluster advance boxes, never glyph ink or inferred character cells.
+    clusters: Vec<FieldCluster>,
+    source_spans: Vec<SourceSpan>,
+    retained_normalized: usize,
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub(super) struct FieldLayout {
+    pub rectangles: Vec<Rectangle>,
+    pub covered: Vec<Range<usize>>,
+    pub truncated: Vec<Range<usize>>,
+    pub unresolved: Vec<UnresolvedField>,
+    pub partially_retained: bool,
+    pub shared_cluster: bool,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(super) enum FieldIssue {
+    MissingGlyph,
+    NoAdvanceGeometry,
+    NonPaintingScalar,
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub(super) struct UnresolvedField {
+    pub source: Range<usize>,
+    pub reason: FieldIssue,
+}
+
+#[derive(Clone, Debug)]
+struct SourceSpan {
+    source: Range<usize>,
+    normalized: Range<usize>,
+    whitespace: bool,
+    non_painting: bool,
+}
+
+#[derive(Clone, Debug)]
+struct FieldCluster {
+    normalized: Range<usize>,
+    rectangles: Vec<Rectangle>,
+    missing: bool,
 }
 
 impl Label {
+    /// Geometry and coverage are separate: truncation is not missing-font
+    /// coverage, and a positive .notdef advance is not represented field text.
+    /// Shared ligatures use whole cluster boxes; no sub-cluster fraction is
+    /// invented. Returned ranges refer only to the requested original source.
+    pub fn field_layout(
+        &self,
+        session: &RenderSession,
+        zoom: f32,
+        position: Point,
+        field: Range<usize>,
+    ) -> Result<FieldLayout, Error> {
+        self.check(session, zoom)?;
+        translated_bounds(position, Size::ZERO)?;
+        if field.start > field.end || self.source.get(field.clone()).is_none() {
+            return Err(Error::InvalidFieldRange);
+        }
+        let mut result = FieldLayout {
+            rectangles: vec![], covered: vec![], truncated: vec![],
+            unresolved: vec![], partially_retained: false, shared_cluster: false,
+        };
+        let mut selected = vec![false; self.clusters.len()];
+        for span in &self.source_spans {
+            if span.whitespace || !overlaps(&span.source, &field) { continue; }
+            // Source spans for text are exact UTF-8 scalars, not intervals
+            // between ends (which could assign trimmed whitespace to text).
+            if span.normalized.end > self.retained_normalized {
+                append_range(&mut result.truncated, span.source.clone());
+                continue;
+            }
+            let matching: Vec<_> = self.clusters.iter().enumerate()
+                .filter(|(_, cluster)| overlaps(&cluster.normalized, &span.normalized))
+                .collect();
+            result.shared_cluster |= matching.iter().any(|(_, cluster)| {
+                cluster.normalized.end > self.retained_normalized
+                    || self.source_spans.iter().any(|other| {
+                        overlaps(&other.normalized, &cluster.normalized)
+                            && !overlaps(&other.source, &field)
+                    })
+            });
+            let issue = if span.non_painting {
+                Some(FieldIssue::NonPaintingScalar)
+            } else if matching.iter().any(|(_, cluster)| cluster.missing) {
+                Some(FieldIssue::MissingGlyph)
+            } else if matching.is_empty() || matching.iter().all(|(_, cluster)| cluster.rectangles.is_empty()) {
+                Some(FieldIssue::NoAdvanceGeometry)
+            } else { None };
+            if let Some(reason) = issue {
+                result.unresolved.push(UnresolvedField { source: span.source.clone(), reason });
+                continue;
+            }
+            append_range(&mut result.covered, span.source.clone());
+            for (index, _) in matching {
+                selected[index] = true;
+            }
+        }
+        for (cluster, selected) in self.clusters.iter().zip(selected) {
+            if !selected { continue; }
+            for bounds in &cluster.rectangles {
+                let translated = translated_bounds(
+                    Point::new(position.x + bounds.x, position.y + bounds.y), bounds.size(),
+                )?;
+                if !result.rectangles.contains(&translated) { result.rectangles.push(translated); }
+            }
+        }
+        result.partially_retained = !result.truncated.is_empty()
+            && (!result.covered.is_empty() || !result.unresolved.is_empty());
+        Ok(result)
+    }
+
     fn check(&self, session: &RenderSession, zoom: f32) -> Result<(), Error> {
         if !Arc::ptr_eq(&self.measured_session.identity, &session.identity) {
             return Err(Error::StaleRenderSession);
@@ -194,6 +309,7 @@ pub(super) enum Error {
     StaleZoom,
     StaleRenderSession,
     FontSetChanged,
+    InvalidFieldRange,
 }
 
 struct Entry {
@@ -286,8 +402,8 @@ impl Cache {
         if cuts.len().saturating_sub(1) > MAX_GRAPHEMES {
             return Err(Error::GraphemeBudget);
         }
-        // Candidate buffers are temporary. Only the resolved strings/metrics
+        // Candidate buffers are temporary. The winning strings, metrics, source
-        // survive, bounded by MAX_ENTRIES; renderer caching remains independent.
+        // spans and cluster boxes survive; renderer caching remains independent.
         let mut shapes = graphics_text::Cache::new();
         let mut measure = |content: &str| -> Result<Size, Error> {
             if content.is_empty() {
@@ -345,6 +461,16 @@ impl Cache {
                 best
             }
         };
+        let clusters = if displayed.is_empty() {
+            // measure("") deliberately creates no cache entry.
+            vec![]
+        } else {
+            let (_, entry) = shapes.allocate(fonts.raw(), graphics_text::cache::Key {
+                content: &displayed, size, line_height, font: style.font,
+                bounds: Size::INFINITE, shaping: style.shaping, align_x: Alignment::Default,
+            });
+            field_clusters(entry, zoom)?
+        };
         let label = Arc::new(Label {
             source: source.into(),
             displayed,
@@ -353,6 +479,9 @@ impl Cache {
             style,
             measured_session: session.clone(),
             measured_zoom: zoom,
+            clusters,
+            source_spans: normalized.spans,
+            retained_normalized: retained,
         });
         if self.entries.len() == MAX_ENTRIES {
             self.entries.pop_front();
@@ -394,12 +523,14 @@ fn translated_bounds(position: Point, size: Size) -> Result<Rectangle, Error> {
 struct Normalized {
     text: String,
     ends: Vec<(usize, usize)>,
+    spans: Vec<SourceSpan>,
 }
 impl Normalized {
     fn new(source: &str) -> Self {
         let mut result = Self {
             text: String::new(),
             ends: vec![(0, 0)],
+            spans: vec![],
         };
         let mut separator = false;
         for (start, c) in source.char_indices() {
@@ -408,11 +539,23 @@ impl Normalized {
                 continue;
             }
             if separator {
+                let normalized_start = result.text.len();
                 result.text.push(' ');
+                result.spans.push(SourceSpan {
+                    source: result.ends.last().expect("initial boundary").1..start,
+                    normalized: normalized_start..result.text.len(),
+                    whitespace: true, non_painting: false,
+                });
                 result.ends.push((result.text.len(), start));
                 separator = false;
             }
+            let normalized_start = result.text.len();
             result.text.push(c);
+            result.spans.push(SourceSpan {
+                source: start..start + c.len_utf8(),
+                normalized: normalized_start..result.text.len(),
+                whitespace: false, non_painting: non_painting_scalar(c),
+            });
             result.ends.push((result.text.len(), start + c.len_utf8()));
         }
         result
@@ -425,6 +568,64 @@ impl Normalized {
     }
 }
 
+fn overlaps(a: &Range<usize>, b: &Range<usize>) -> bool {
+    a.start < a.end && b.start < b.end && a.start < b.end && b.start < a.end
+}
+
+fn append_range(ranges: &mut Vec<Range<usize>>, next: Range<usize>) {
+    if let Some(last) = ranges.last_mut()
+        && last.end == next.start {
+        last.end = next.end;
+        return;
+    }
+    ranges.push(next);
+}
+
+// Conservative non-membership for controls/default-ignorables, even when a
+// shaper assigns them a neighbor's positive-width cluster. Unicode 14 ranges
+// documented in pinned harfrust 0.3.2 src/hb/unicode.rs:615–650. Include its
+// spacing-filler exceptions conservatively; this is not an ink classifier.
+fn non_painting_scalar(c: char) -> bool {
+    c.is_control() || matches!(c as u32,
+        0x00ad | 0x034f | 0x061c | 0x115f..=0x1160 | 0x17b4..=0x17b5 |
+        0x180b..=0x180f | 0x200b..=0x200f | 0x202a..=0x202e | 0x2060..=0x206f |
+        0x3164 | 0xfe00..=0xfe0f | 0xfeff | 0xffa0 | 0xfff0..=0xfff8 |
+        0x1bca0..=0x1bca3 | 0x1d173..=0x1d17a | 0xe0000..=0xe0fff)
+}
+
+fn field_clusters(entry: &graphics_text::cache::Entry, zoom: f32) -> Result<Vec<FieldCluster>, Error> {
+    let mut clusters: std::collections::BTreeMap<(usize, usize), FieldCluster> = Default::default();
+    for run in entry.buffer.layout_runs() {
+        for glyph in run.glyphs {
+            if glyph.start >= glyph.end || run.text.get(glyph.start..glyph.end).is_none() {
+                return Err(Error::InvalidMeasurement);
+            }
+            let cluster = clusters.entry((glyph.start, glyph.end)).or_insert_with(|| FieldCluster {
+                normalized: glyph.start..glyph.end, rectangles: vec![], missing: false,
+            });
+            cluster.missing |= glyph.glyph_id == 0;
+            let x1 = glyph.x / zoom;
+            let x2 = (glyph.x + glyph.w) / zoom;
+            let y = (run.line_top + glyph.y) / zoom;
+            let height = glyph.line_height_opt.unwrap_or(run.line_height) / zoom;
+            if !x1.is_finite() || !x2.is_finite() || !y.is_finite()
+                || !height.is_finite() || height < 0. || !(y + height).is_finite() {
+                return Err(Error::InvalidMeasurement);
+            }
+            if x1 != x2 && height > 0. {
+                let bounds = Rectangle::new(Point::new(x1.min(x2), y), Size::new((x2 - x1).abs(), height));
+                if !bounds.width.is_finite() { return Err(Error::InvalidMeasurement); }
+                if !cluster.rectangles.contains(&bounds) { cluster.rectangles.push(bounds); }
+            }
+        }
+    }
+    Ok(clusters.into_values().collect())
+}
+
 #[cfg(test)]
 #[path = "scenario_label/tests.rs"]
 mod tests;
+
+#[cfg(test)]
+#[path = "scenario_label/field_tests.rs"]
+mod field_tests;

crates/oa-desktop/src/behavior/scenario_label/field_tests.rs

@@ -0,0 +1,230 @@
+//! Unwired, unexecuted field-layout falsifiers. Paragraph is an independent
+//! shaping oracle; it never invokes the candidate normalization/extraction.
+use super::*;
+use iced::advanced::{
+    graphics::text::Paragraph,
+    text::{Paragraph as _, Text as ParagraphText, Wrapping},
+};
+
+fn paragraph(text: &str, zoom: f32, style: Style) -> Paragraph {
+    Paragraph::with_text(ParagraphText {
+        content: text, bounds: Size::INFINITE, size: Pixels(style.size.0 * zoom),
+        line_height: match style.line_height {
+            LineHeight::Relative(value) => LineHeight::Relative(value),
+            LineHeight::Absolute(value) => LineHeight::Absolute(Pixels(value.0 * zoom)),
+        },
+        font: style.font, align_x: Alignment::Default,
+        align_y: iced::alignment::Vertical::Top, shaping: style.shaping,
+        wrapping: Wrapping::default(),
+    })
+}
+
+fn resolve(text: &str, width: f32, zoom: f32, style: Style) -> (RenderSession, Arc<Label>) {
+    let session = RenderSession::after_fonts_settled_and_renderer_reset().unwrap();
+    let label = Cache::default().resolve(&session, text, width, zoom, style).unwrap();
+    (session, label)
+}
+
+#[test]
+fn empty_and_ellipsis_only_have_no_field_coverage() {
+    for text in ["", " \t\n", "Operation: assess"] {
+        let (session, label) = resolve(text, 0., 1., Style::default());
+        assert!(label.displayed.is_empty());
+        let field = label.field_layout(&session, 1., Point::ORIGIN, 0..text.len()).unwrap();
+        assert!(field.rectangles.is_empty());
+        assert!(field.covered.is_empty());
+        assert!(field.unresolved.is_empty());
+    }
+    let style = Style::default();
+    let width = paragraph("…", 1., style).min_width();
+    let text = "Operation: assess";
+    let (session, label) = resolve(text, width, 1., style);
+    assert_eq!(label.displayed, "…", "must exercise synthetic-only displayed content");
+    let field = label.field_layout(&session, 1., Point::ORIGIN, 0..text.len()).unwrap();
+    assert!(field.covered.is_empty());
+    assert!(field.rectangles.is_empty());
+    assert!(!field.truncated.is_empty());
+}
+
+#[test]
+fn exact_scalar_mapping_does_not_assign_trimmed_or_collapsed_whitespace_to_text() {
+    let source = "  send\t\n  assess  ";
+    let (session, label) = resolve(source, 900., 1., Style::default());
+    assert_eq!(label.displayed, "send assess");
+    for range in [0..2, 6..10, 16..18] {
+        let result = label.field_layout(&session, 1., Point::ORIGIN, range).unwrap();
+        assert!(result.covered.is_empty());
+        assert!(result.rectangles.is_empty());
+        assert!(result.unresolved.is_empty());
+    }
+    let result = label.field_layout(&session, 1., Point::ORIGIN, 10..16).unwrap();
+    assert_eq!(result.covered, vec![10..16]);
+    assert!(!result.rectangles.is_empty());
+    assert!(result.truncated.is_empty());
+    assert!(result.unresolved.is_empty());
+}
+
+#[test]
+fn late_operation_reports_hidden_partial_and_complete_without_ellipsis_membership() {
+    let source = "1. call - Incident report - assess_pressure";
+    let start = source.find("assess_pressure").unwrap();
+    let style = Style { shaping: Shaping::Advanced, ..Style::default() };
+    let (session, full) = resolve(source, 900., 1., style);
+    let complete = full.field_layout(&session, 1., Point::ORIGIN, start..source.len()).unwrap();
+    assert_eq!(complete.covered, vec![start..source.len()]);
+    assert!(!complete.partially_retained);
+    let (_, hidden) = resolve(source, paragraph("1. call…", 1., style).min_width(), 1., style);
+    let hidden_result = hidden.field_layout(&hidden.measured_session, 1., Point::ORIGIN, start..source.len()).unwrap();
+    assert!(hidden.retained_source.end <= start);
+    assert!(hidden_result.covered.is_empty());
+    assert_eq!(hidden_result.truncated, vec![start..source.len()]);
+    assert!(!hidden_result.partially_retained);
+    let desired = format!("{}assess…", &source[..start]);
+    let (_, partial) = resolve(source, paragraph(&desired, 1., style).min_width(), 1., style);
+    assert!(partial.retained_source.end > start && partial.retained_source.end < source.len(),
+        "actual fitting result must retain a strict operation prefix");
+    let result = partial.field_layout(&partial.measured_session, 1., Point::ORIGIN, start..source.len()).unwrap();
+    assert_eq!(result.covered, vec![start..partial.retained_source.end]);
+    assert_eq!(result.truncated, vec![partial.retained_source.end..source.len()]);
+    assert!(result.partially_retained);
+    assert!(result.unresolved.is_empty());
+}
+
+#[test]
+fn positive_notdef_advance_is_unresolved_not_covered_or_truncated() {
+    let source = "A\u{10ffff}B";
+    let style = Style { shaping: Shaping::Advanced, ..Style::default() };
+    let oracle = paragraph(source, 1., style);
+    assert!(oracle.buffer().layout_runs().flat_map(|run| run.glyphs)
+        .any(|glyph| glyph.start == 1 && glyph.end == 5 && glyph.glyph_id == 0 && glyph.w > 0.),
+        "font precondition: actual positive-width .notdef for the noncharacter");
+    let (session, label) = resolve(source, 900., 1., style);
+    let missing = label.field_layout(&session, 1., Point::ORIGIN, 1..5).unwrap();
+    assert!(missing.covered.is_empty());
+    assert!(missing.rectangles.is_empty());
+    assert!(missing.truncated.is_empty());
+    assert_eq!(missing.unresolved, vec![UnresolvedField { source: 1..5, reason: FieldIssue::MissingGlyph }]);
+    let mixed = label.field_layout(&session, 1., Point::ORIGIN, 0..source.len()).unwrap();
+    assert_eq!(mixed.covered, vec![0..1, 5..6]);
+    assert_eq!(mixed.unresolved, missing.unresolved);
+    assert!(!mixed.rectangles.is_empty());
+}
+
+#[test]
+fn advanced_ligature_partial_field_uses_whole_shared_cluster_not_fraction() {
+    let source = "ffi";
+    let style = Style { shaping: Shaping::Advanced, ..Style::default() };
+    let oracle = paragraph(source, 1., style);
+    let run = oracle.buffer().layout_runs().next().unwrap();
+    let glyph = run.glyphs.iter().find(|glyph| glyph.start == 0 && glyph.end == 3 && glyph.w > 0. && glyph.glyph_id != 0)
+        .expect("font precondition: Advanced must produce the ffi ligature cluster");
+    let expected = Rectangle::new(Point::new(glyph.x, run.line_top + glyph.y),
+        Size::new(glyph.w, glyph.line_height_opt.unwrap_or(run.line_height)));
+    let (session, label) = resolve(source, 900., 1., style);
+    let field = label.field_layout(&session, 1., Point::ORIGIN, 1..2).unwrap();
+    assert_eq!(field.covered, vec![1..2]);
+    assert!(field.shared_cluster);
+    assert!(field.rectangles.contains(&expected));
+    assert_eq!(field.rectangles.iter().map(|rect| rect.width).sum::<f32>(), expected.width);
+    assert!(field.unresolved.is_empty());
+}
+
+#[test]
+fn shared_default_ignorable_does_not_inherit_neighbor_coverage() {
+    let source = "a\u{fe0f}";
+    let style = Style { shaping: Shaping::Advanced, ..Style::default() };
+    let oracle = paragraph(source, 1., style);
+    assert!(oracle.buffer().layout_runs().flat_map(|run| run.glyphs)
+        .any(|glyph| glyph.start == 0 && glyph.end == source.len() && glyph.w > 0. && glyph.glyph_id != 0),
+        "precondition: variation selector shares a real positive-width base cluster");
+    let (session, label) = resolve(source, 900., 1., style);
+    let field = label.field_layout(&session, 1., Point::ORIGIN, 1..source.len()).unwrap();
+    assert!(field.covered.is_empty());
+    assert!(field.rectangles.is_empty());
+    assert!(field.truncated.is_empty());
+    assert!(field.shared_cluster);
+    assert_eq!(field.unresolved, vec![UnresolvedField { source: 1..source.len(), reason: FieldIssue::NonPaintingScalar }]);
+}
+
+#[test]
+fn combining_mark_field_retains_cluster_geometry_without_splitting_the_grapheme() {
+    let source = "e\u{301} next";
+    let style = Style { shaping: Shaping::Advanced, ..Style::default() };
+    let oracle = paragraph(source, 1., style);
+    let run = oracle.buffer().layout_runs().next().unwrap();
+    let cluster = run.glyphs.iter().find(|glyph| glyph.start == 0 && glyph.end == 3 && glyph.w > 0. && glyph.glyph_id != 0)
+        .expect("precondition: composed accent and base share the grapheme cluster");
+    let (session, label) = resolve(source, 900., 1., style);
+    let field = label.field_layout(&session, 1., Point::ORIGIN, 1..3).unwrap();
+    assert_eq!(field.covered, vec![1..3]);
+    assert!(field.shared_cluster);
+    assert!(field.rectangles.iter().any(|rect| rect.x == cluster.x && rect.width == cluster.w));
+    assert!(field.unresolved.is_empty());
+    assert_eq!(label.field_layout(&session, 1., Point::ORIGIN, 2..3), Err(Error::InvalidFieldRange));
+    let (_, short) = resolve(source, paragraph("e\u{301}…", 1., style).min_width(), 1., style);
+    assert_ne!(short.retained_source.end, 1, "the chosen prefix must never split the combining grapheme");
+}
+
+#[test]
+fn bidi_logical_field_keeps_visual_pieces_without_enclosing_unselected_glyph() {
+    let source = "abc אבג xyz";
+    let style = Style { shaping: Shaping::Advanced, ..Style::default() };
+    let oracle = paragraph(source, 1., style);
+    let run = oracle.buffer().layout_runs().next().unwrap();
+    assert!(run.glyphs.iter().any(|glyph| glyph.level.is_rtl()));
+    assert!(run.glyphs.iter().any(|glyph| !glyph.level.is_rtl()));
+    let excluded = run.glyphs.iter().find(|glyph| glyph.start == 8 && glyph.end == 10 && glyph.w > 0. && glyph.glyph_id != 0).unwrap();
+    let (session, label) = resolve(source, 900., 1., style);
+    let field = label.field_layout(&session, 1., Point::ORIGIN, 1..8).unwrap();
+    assert_eq!(field.covered, vec![1..3, 4..8]);
+    assert!(field.unresolved.is_empty());
+    assert!(field.rectangles.iter().any(|rect| rect.x + rect.width <= excluded.x));
+    assert!(field.rectangles.iter().any(|rect| rect.x >= excluded.x + excluded.w));
+    assert!(field.rectangles.iter().all(|rect|
+        rect.x + rect.width <= excluded.x || rect.x >= excluded.x + excluded.w),
+        "logical selection must not become one envelope over the intervening visual glyph");
+}
+
+#[test]
+fn auto_ascii_to_ellipsis_advanced_transition_uses_final_buffer() {
+    let style = Style { shaping: Shaping::Auto, ..Style::default() };
+    let source = "ffi office assessment";
+    assert_eq!(graphics_text::to_shaping(style.shaping, source), graphics_text::cosmic_text::Shaping::Basic);
+    let (session, label) = resolve(source, paragraph("ffi office…", 1., style).min_width(), 1., style);
+    assert!(label.displayed.ends_with('…'));
+    assert_eq!(graphics_text::to_shaping(style.shaping, &label.displayed), graphics_text::cosmic_text::Shaping::Advanced);
+    let oracle = paragraph(&label.displayed, 1., style);
+    let run = oracle.buffer().layout_runs().next().unwrap();
+    let first = run.glyphs.iter().find(|glyph| glyph.start == 0 && glyph.end >= 3 && glyph.w > 0. && glyph.glyph_id != 0)
+        .expect("final Advanced shape must expose the ligature precondition");
+    let field = label.field_layout(&session, 1., Point::ORIGIN, 0..3).unwrap();
+    assert_eq!(field.covered, vec![0..3]);
+    assert!(field.rectangles.iter().any(|rect| rect.x == first.x && rect.width == first.w));
+}
+
+#[test]
+fn field_geometry_reuses_zoom_session_guards_and_translates_actual_cluster_boxes() {
+    let source = "Z";
+    for zoom in [0.5, 1.75] {
+        let style = Style::default();
+        let oracle = paragraph(source, zoom, style);
+        let run = oracle.buffer().layout_runs().next().unwrap();
+        assert_eq!(run.glyphs.len(), 1);
+        let glyph = &run.glyphs[0];
+        assert!(glyph.glyph_id != 0 && glyph.w > 0.);
+        let (session, label) = resolve(source, 900., zoom, style);
+        let position = Point::new(-300., 90.);
+        let field = label.field_layout(&session, zoom, position, 0..1).unwrap();
+        assert_eq!(field.rectangles, vec![Rectangle::new(
+            Point::new(position.x + glyph.x / zoom, position.y + (run.line_top + glyph.y) / zoom),
+            Size::new(((glyph.x + glyph.w) / zoom - glyph.x / zoom).abs(), glyph.line_height_opt.unwrap_or(run.line_height) / zoom),
+        )]);
+        assert_eq!(label.field_layout(&session, zoom + 0.1, position, 0..1), Err(Error::StaleZoom));
+        let fresh = RenderSession::after_fonts_settled_and_renderer_reset().unwrap();
+        assert_eq!(label.field_layout(&fresh, zoom, position, 0..1), Err(Error::StaleRenderSession));
+        assert_eq!(label.field_layout(&session, zoom, Point::new(f32::NAN, 0.), 0..1), Err(Error::InvalidPosition));
+        assert_eq!(label.field_layout(&session, zoom, position, 0..2), Err(Error::InvalidFieldRange));
+        let empty = label.field_layout(&session, zoom, position, 0..0).unwrap();
+        assert!(empty.covered.is_empty() && empty.rectangles.is_empty());
+    }
+}