Preserve scenario viewport when editing a diagram message

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

@@ -1221,8 +1221,11 @@ pub fn view<'a>(app: &'a App, owner: Id, mut out: Column<'a, Message>) -> Elemen
         }
     }));
     let pending = app.behavior.has_pending();
+    // Keep the canvas at a stable widget-tree position when a form replaces
+    // the palette, so Iced retains the user's current sequence viewport.
+    let mut controls = column![].spacing(12);
     if !pending {
-        out = out
+        controls = controls
             .push(
                 row![
                     action("+ Lifeline", event(Event::Start(owner, Tool::Lifeline))),
@@ -1252,7 +1255,7 @@ pub fn view<'a>(app: &'a App, owner: Id, mut out: Column<'a, Message>) -> Elemen
         MessageStyle::CallReply
     };
     if !pending {
-        out = out.push(
+        controls = controls.push(
             row![
                 button("Select / reorder")
                     .on_press(Message::ScenarioCanvas(CanvasEvent::Choose(
@@ -1296,14 +1299,15 @@ pub fn view<'a>(app: &'a App, owner: Id, mut out: Column<'a, Message>) -> Elemen
             ]
             .spacing(6),
         );
-        out = out.push(label(if matches!(tool, CanvasTool::Message(_)) {
+        controls = controls.push(label(if matches!(tool, CanvasTool::Message(_)) {
         "Draw message · choose the highlighted sequence position on a lifeline, then click or drag to another lifeline · Escape cancels"
     } else { "Select a lifeline or occurrence · drag the selected occurrence’s ↕ handle to reorder within its sequence" }));
     } else {
-        out = out.push(label(
+        controls = controls.push(label(
             "Pending form · the canvas remains visible · apply or cancel below",
         ));
     }
+    out = out.push(controls);
     let mut scene = Scene::sequence(p, owner, app.selected);
     scene.interaction = Some((
         context,