From 80cf197be6f1d400bf0b9c84bf8557f88c895aa6 Mon Sep 17 00:00:00 2001
From: Codex <codex@localhost>
Date: Fri, 11 Sep 2026 01:27:48 -0700
Subject: [PATCH] Remeasure scenario Fit viewport after owner or document
 changes

---
 .../src/behavior/scenario/workspace.rs        |  1 +
 .../src/behavior/scenario/workspace/tests.rs  | 66 +++++++++++++++++++
 2 files changed, 67 insertions(+)

diff --git a/crates/oa-desktop/src/behavior/scenario/workspace.rs b/crates/oa-desktop/src/behavior/scenario/workspace.rs
index ea63b579..121136ce 100644
--- a/crates/oa-desktop/src/behavior/scenario/workspace.rs
+++ b/crates/oa-desktop/src/behavior/scenario/workspace.rs
@@ -649,6 +649,7 @@ fn canvas_pane<'a, R: iced::advanced::text::Renderer + 'a>(
                 .width(Fill)
                 .height(Fill),
         ))
+        .key((context.document, context.scenario))
         .on_show(move |size| Message::ScenarioCanvas(CanvasEvent::Viewport(context, size)))
         .on_resize(move |size| Message::ScenarioCanvas(CanvasEvent::Viewport(context, size))),
     )
diff --git a/crates/oa-desktop/src/behavior/scenario/workspace/tests.rs b/crates/oa-desktop/src/behavior/scenario/workspace/tests.rs
index 8829a024..8c32bfa7 100644
--- a/crates/oa-desktop/src/behavior/scenario/workspace/tests.rs
+++ b/crates/oa-desktop/src/behavior/scenario/workspace/tests.rs
@@ -341,6 +341,72 @@ fn scenario_workspace_fit_uses_measured_inner_viewport_and_rejects_nonfinite_inp
     );
 }
 
+#[test]
+fn scenario_workspace_same_size_owner_and_document_changes_remeasure_fit_viewport() {
+    use super::super::direct_tests::{navigation_event, navigation_fixture};
+    let (project, [a, b, _], [reference, _, _]) = navigation_fixture();
+    let mut app = App::with_project(project.clone());
+    let _ = app.update(Message::Select(a));
+    let _ = app.update(Message::Page(crate::app::Page::Behavior));
+    let size = Size::new(500., 570.);
+    let graph = Size::new(1800., 2400.);
+    let mut view = fixture_view(
+        &app.behavior.scenario,
+        app.scenario_context().unwrap(),
+        false,
+        68.,
+    );
+    let mut tree = widget::Tree::new(view.as_widget());
+    redraw(&mut view, &mut tree, size, &mut app);
+    let first_viewport = app.behavior.scenario.presentation.viewport.unwrap();
+    navigation_event(&mut app, |ctx| CanvasEvent::Zoom(ctx, None));
+    assert!(app.behavior.scenario.presentation.scale(graph) < 1.);
+
+    navigation_event(&mut app, |ctx| CanvasEvent::OpenUse(ctx, reference));
+    assert_eq!(app.scenario_context().unwrap().scenario, b);
+    assert!(app.behavior.scenario.presentation.viewport.is_none());
+    view = fixture_view(
+        &app.behavior.scenario,
+        app.scenario_context().unwrap(),
+        false,
+        68.,
+    );
+    tree.diff(view.as_widget());
+    redraw(&mut view, &mut tree, size, &mut app);
+    assert_eq!(
+        app.behavior.scenario.presentation.viewport,
+        Some(first_viewport)
+    );
+    navigation_event(&mut app, |ctx| CanvasEvent::Zoom(ctx, None));
+    let fitted = app.behavior.scenario.presentation.scale(graph);
+    assert!(graph.width * fitted <= first_viewport.width - 16.);
+    assert!(graph.height * fitted <= first_viewport.height - 16.);
+
+    let prior_document = app.scenario_context().unwrap().document;
+    app = App::with_project(project);
+    let _ = app.update(Message::Select(b));
+    let _ = app.update(Message::Page(crate::app::Page::Behavior));
+    let context = app.scenario_context().unwrap();
+    assert_ne!(context.document, prior_document);
+    assert_eq!(
+        context.scenario, b,
+        "semantic UUID is retained across documents"
+    );
+    assert!(app.behavior.scenario.presentation.viewport.is_none());
+    let accepted = app.document.snapshot();
+    view = fixture_view(&app.behavior.scenario, context, false, 68.);
+    tree.diff(view.as_widget());
+    redraw(&mut view, &mut tree, size, &mut app);
+    assert_eq!(
+        app.behavior.scenario.presentation.viewport,
+        Some(first_viewport)
+    );
+    navigation_event(&mut app, |ctx| CanvasEvent::Zoom(ctx, None));
+    assert_eq!(app.behavior.scenario.presentation.scale(graph), fitted);
+    assert!(std::sync::Arc::ptr_eq(&accepted, &app.document.snapshot()));
+    assert!(!app.document.can_undo());
+}
+
 #[test]
 fn scenario_workspace_open_and_back_restore_owner_and_mounted_scroll_after_preselection() {
     use super::super::direct_tests::{navigation_event, navigation_fixture};