feat: draw a notice for one frame before rebuilding a layout

Switching to a Layout stops the main thread long enough that the
compositor decides the application has stopped responding and offers to
kill it. The switch itself is not the problem — `layout-switch` reports
about 10 ms — it is the frame after it, which rebuilds the whole wire set.

That frame now has a cheap one in front of it. `layout_settling` is set
when the switch takes, and while it holds, the viewport is replaced by a
centered "Preparing layout…" on the canvas background. No shader widget
means no scene primitive, so `prepare` has nothing to do and the frame
costs nothing. A `window::frames` subscription clears the flag, which is
the guarantee this needs: that callback runs once a frame has actually
been presented, so the notice is on screen and the compositor has been
answered before the expensive frame starts.

The deferral is of the *frame*, not the switch. Two callers depend on the
switch having completed when it returns — one calls `fit_all` immediately
after, another compares `current_layout` — so deferring the switch itself
would have broken both.

This restarts the compositor's timer immediately before the slow frame and
replaces a stale window with something that says what is happening. It
does not make the slow frame fast: if the rebuild still exceeds the
compositor's timeout the dialog can return, and the durable fix is the
2 s `memo_miss` behind it, which is a separate change.

Only `en-US` is validated against at build time; Greek is filled in too
since it is in use here, and the rest fall back until translated.

Co-Authored-By: Claude Opus 5 <[email protected]>

locales/el-GR/opencadstudio.ftl

@@ -167,6 +167,7 @@ clipboard =
     .paste-to-original-coordinates = Επικόλληση στις αρχικές συντεταγμένες
 
 viewport =
+    .preparing-layout = Προετοιμασία διάταξης…
     .split-vertical = Κατακόρυφος διαχωρισμός θύρας προβολής
     .split-horizontal = Οριζόντιος διαχωρισμός θύρας προβολής
     .move = Μετακίνηση θύρας προβολής

locales/en-US/opencadstudio.ftl

@@ -167,6 +167,7 @@ clipboard =
     .paste-to-original-coordinates = Paste to Original Coordinates
 
 viewport =
+    .preparing-layout = Preparing layout…
     .split-vertical = Split Viewport Vertically
     .split-horizontal = Split Viewport Horizontally
     .move = Move Viewport

src/app/mod.rs

@@ -1055,6 +1055,15 @@ pub(super) struct OpenCADStudio {
     /// `Some` while a CAD file is loading — drives the modal overlay.
     /// Cleared when the load finishes, errors, or the user cancels.
     pub(super) opening: Option<OpenProgress>,
+    /// Set the moment a layout switch changes `current_layout`, cleared on the
+    /// next presented frame.
+    ///
+    /// The switch itself costs about 10 ms; the frame after it rebuilds the
+    /// whole wire set and can block the main thread for seconds. While this is
+    /// set the viewport is replaced by a notice, so that frame is cheap — which
+    /// gives the compositor an answer, and the user something other than a
+    /// stale window, immediately before the expensive one.
+    pub(super) layout_settling: bool,
     open_job_serial: u64,
     /// Last repair or failed-open report shown in the recovery modal.
     recovery_report: Option<crate::io::recovery::RecoveryReport>,
@@ -2283,6 +2292,9 @@ pub enum Message {
     /// Timer pulse while the cursor is dwelling on a grip; drives the
     /// dwell-to-popup transition without requiring further mouse motion.
     GripDwellTick,
+    /// One frame has been presented since a layout switch; the next may do the
+    /// expensive rebuild.
+    LayoutSettled,
     /// Timer pulse while a rollover hit-test is queued; fires when the
     /// cursor has been still long enough to safely run the pick.
     HoverDwellTick,
@@ -3484,6 +3496,7 @@ impl OpenCADStudio {
             print_all_plot_window_prev: None,
             print_all_plot_setup_prev: None,
             opening: None,
+            layout_settling: false,
             open_job_serial: 0,
             recovery_report: None,
             pending_opens: std::collections::VecDeque::new(),

src/app/update/mod.rs

@@ -3045,6 +3045,12 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::LayoutSettled => {
+                // The notice frame has been presented, so the compositor has
+                // had its answer. Let the next frame build the real thing.
+                self.layout_settling = false;
+                Task::none()
+            }
             Message::GripDwellTick => {
                 let i = self.active_tab;
                 // Reuse the move-time logic — `p` is the last cursor

src/app/update/viewport.rs

@@ -5534,6 +5534,10 @@ impl OpenCADStudio {
             .layers
             .sync_with_viewports(&doc_layers, vp_info);
         let layers_ms = perf_phase.elapsed().as_secs_f64() * 1000.0;
+        // The switch is cheap; the frame that follows it is not. Draw a notice
+        // for one frame first, so the compositor gets an answer before the main
+        // thread disappears into rebuilding the wire set.
+        self.layout_settling = true;
         if perf {
             crate::perf_record!(
                 "[perf] layout-switch from={} to={} total={:.2}ms sync={:.2}ms switch={:.2}ms restore={:.2}ms layers={:.2}ms epoch={}",

src/app/view/mod.rs

@@ -272,7 +272,33 @@ impl OpenCADStudio {
         // viewport draws the layout's own geometry (white sheet + entities +
         // borders) and the floating content viewports blit on top.
         mark("viewport_3d");
-        let viewport_3d: Element<'_, Message> = if tab.is_start {
+        let viewport_3d: Element<'_, Message> = if self.layout_settling && !tab.is_start {
+            // One cheap frame between the switch and the rebuild. No shader
+            // widget means no scene primitive, so `prepare` has nothing to do
+            // and the event loop turns — which is what stops the compositor
+            // deciding the application has stopped responding.
+            let bg = crosshair_background(tab, is_paper);
+            let lum = 0.299 * bg[0] + 0.587 * bg[1] + 0.114 * bg[2];
+            let ink = if lum > 0.5 {
+                Color::from_rgba(0.0, 0.0, 0.0, 0.55)
+            } else {
+                Color::from_rgba(1.0, 1.0, 1.0, 0.55)
+            };
+            container(
+                text(crate::tr!("viewport", "preparing-layout"))
+                    .size(15)
+                    .color(ink),
+            )
+            .width(Fill)
+            .height(Fill)
+            .center_x(Fill)
+            .center_y(Fill)
+            .style(move |_| container::Style {
+                background: Some(Color::from_rgba(bg[0], bg[1], bg[2], bg[3]).into()),
+                ..container::Style::default()
+            })
+            .into()
+        } else if tab.is_start {
             start_page_view(
                 &self.patrons,
                 &self.videos,
@@ -2253,6 +2279,15 @@ impl OpenCADStudio {
         // the mouse perfectly still — `ViewportMove` alone would never
         // fire again. Auto-stops once the hover clears or the popup is
         // already open.
+        // One frame after a layout switch, then stop. `window::frames` fires
+        // once a frame has actually been presented, which is the guarantee this
+        // needs: the notice is on screen and the compositor has been answered
+        // before the expensive frame starts.
+        let layout_settle = if self.layout_settling {
+            window::frames().map(|_| Message::LayoutSettled)
+        } else {
+            Subscription::none()
+        };
         let grip_dwell = if self.grip_hover.is_some() && self.grip_popup.is_none() {
             window::frames().map(|_| Message::GripDwellTick)
         } else {
@@ -2478,6 +2513,7 @@ impl OpenCADStudio {
             control,
             frames,
             history_tick,
+            layout_settle,
             grip_dwell,
             hover_dwell,
             nav_settle,