Keep diagram viewport stable when modal remounts canvas

crates/oa-desktop/src/app.rs

@@ -1468,8 +1468,11 @@ impl App {
             Message::WindowResized(size) => self.window_size = size,
             Message::CanvasResized(size) => {
                 let first_measurement = self.canvas_size.is_none();
+                let size_changed = self.canvas_size != Some(size);
                 self.canvas_size = Some(size);
-                if first_measurement || self.fit_to_view {
+                // Modal overlays can remount the sensor and repeat on_show at
+                // the same size. Keep the viewport stable after model edits.
+                if first_measurement || (size_changed && self.fit_to_view) {
                     self.fit();
                 }
             }

crates/oa-desktop/src/tests.rs

@@ -171,6 +171,57 @@ fn fit_adapts_to_canvas_returning_after_help_but_preserves_manual_navigation() {
     assert_eq!((app.zoom, app.pan), manual);
 }
 
+#[test]
+fn reopening_canvas_for_a_modal_preserves_viewport_after_route_edits() {
+    let mut p = Project::empty();
+    let sender = p.add(Kind::Activity, "Report", 0., 0.);
+    let target = p.add(Kind::Activity, "Receive", 600., 0.);
+    let edge = p.relate(RelationKind::Interaction, "Report signal", sender, target);
+    let diagram = p.ensure_diagram(Layer::Oa);
+    let mut app = App::with_project(p);
+    let canvas = iced::Size::new(900., 600.);
+    send(&mut app, Message::CanvasResized(canvas));
+    let viewport = (app.zoom, app.pan);
+    send(
+        &mut app,
+        Message::Command(Command::SetEdgeRoute {
+            diagram,
+            route: EdgeRoute {
+                edge,
+                waypoints: vec![Position { x: 400., y: 400. }],
+                label: Some(EdgeLabel {
+                    fraction: 0.5,
+                    offset: Position { x: 0., y: 150. },
+                }),
+            },
+        }),
+    );
+    assert_eq!((app.zoom, app.pan), viewport);
+    let mut fitted = App::with_project(app.project().clone());
+    send(&mut fitted, Message::CanvasResized(canvas));
+    assert_ne!(
+        (fitted.zoom, fitted.pan),
+        viewport,
+        "The edited route must change the fitted bounds to expose an accidental refit"
+    );
+
+    // The shared file modal remounts the canvas sensor. Its on_show measurement
+    // describes the existing viewport, not a user request to fit the edited model.
+    send(&mut app, Message::Export);
+    assert!(app.file_prompt.is_some());
+    send(&mut app, Message::CanvasResized(canvas));
+    assert_eq!((app.zoom, app.pan), viewport);
+    send(&mut app, Message::CancelFile);
+    send(&mut app, Message::CanvasResized(canvas));
+    assert_eq!((app.zoom, app.pan), viewport);
+
+    let compact = iced::Size::new(650., 400.);
+    send(&mut app, Message::CanvasResized(compact));
+    send(&mut fitted, Message::CanvasResized(compact));
+    assert_eq!((app.zoom, app.pan), (fitted.zoom, fitted.pan));
+    assert_ne!((app.zoom, app.pan), viewport);
+}
+
 #[test]
 fn source_navigation_waits_for_editor_layout_and_cancels_on_edit() {
     let p = fixture();