fix(hover): defer rollover pick until the cursor stops

The rollover hit-test now sweeps wires, hatches, block-internal hatches
and shaded solid bodies, which is O(N) per cursor frame and was freezing
the crosshair on large drawings. Each idle move now clears the live
highlight and arms a dwell timer; HoverDwellTick runs the pick once the
cursor has been still for HOVER_DWELL_MS (120 ms), using the active
tile's bounds so the deferred pick matches the entity actually under
the cursor in tiled viewports.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/mod.rs

@@ -48,6 +48,23 @@ pub struct GripHover {
     pub started: iced::time::Instant,
 }
 
+/// Cursor dwell awaiting a rollover hit-test. Refreshed on every idle
+/// move; `HoverDwellTick` runs the pick once `last_move_at.elapsed()`
+/// crosses `HOVER_DWELL_MS`. `point` and `tile_size` are tile-local so
+/// the deferred pick uses the same projection the move handler would
+/// have — picking with the full canvas bounds in a tiled layout matches
+/// the wrong entity under the cursor.
+#[derive(Clone, Debug)]
+pub struct HoverDwell {
+    pub last_move_at: iced::time::Instant,
+    pub point: iced::Point,
+    pub tile_size: (f32, f32),
+    pub tab: usize,
+}
+
+/// How long the cursor must sit still before the idle rollover pick runs.
+pub const HOVER_DWELL_MS: u128 = 120;
+
 /// Open multi-functional-grip popup state.
 #[derive(Clone, Debug)]
 pub struct GripPopup {
@@ -259,6 +276,12 @@ pub(super) struct OpenCADStudio {
     /// updates only the overlay rather than re-tessellating the whole model.
     /// Committed (un-hidden + one re-tess) when the drag ends. `None` = idle.
     grip_preview_handle: Option<acadrust::Handle>,
+    /// Pending rollover hit-test. Each idle cursor move stashes
+    /// `(last_move_at, point, tab)` here and clears the live highlight;
+    /// `HoverDwellTick` runs the pick once the cursor has been still for
+    /// `HOVER_DWELL_MS`. Skipping the pick mid-stroke avoids the per-frame
+    /// O(N) wire+hatch+mesh sweep that froze the cursor on large drawings.
+    hover_dwell: Option<HoverDwell>,
     /// Snapshot of the edited entity taken at the start of a grip drag, used to
     /// restore it if the user presses Esc to cancel the drag. The drag mutates
     /// the document live (so grips / properties track), so cancel reverts from
@@ -943,6 +966,9 @@ 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,
+    /// Timer pulse while a rollover hit-test is queued; fires when the
+    /// cursor has been still long enough to safely run the pick.
+    HoverDwellTick,
     WindowResized(f32, f32),
     /// Enter pressed globally — finalises the active command (no text-input involvement).
     CommandFinalize,
@@ -1584,6 +1610,7 @@ impl OpenCADStudio {
             visibility_popup: None,
             grip_add_provisional: None,
             grip_preview_handle: None,
+            hover_dwell: None,
             grip_original: None,
             qselect: None,
             show_ucs_icon: true,

src/app/update.rs

@@ -2709,45 +2709,25 @@ impl OpenCADStudio {
                     self.tabs[i].last_cursor_world = world;
                 }
 
-                // Rollover highlight: when idle (no active command, no drag),
-                // highlight the entity under the cursor. Decoupling selection
-                // from tessellation makes this cheap — it only refreshes the GPU
-                // xray overlay via `bump_selection`, never re-tessellates.
+                // Rollover highlight: when idle (no active command, no
+                // drag), defer the pick until the cursor stops. The full
+                // pick (wires + hatches + block hatches + shaded meshes) is
+                // O(N) per frame and stalls the cursor on large drawings,
+                // so each move clears the current highlight and resets the
+                // dwell timer — `HoverDwellTick` runs the hit-test only
+                // once the cursor has been still for `HOVER_DWELL_MS`.
                 if !dragging && self.tabs[i].active_cmd.is_none() {
-                    let bounds = iced::Rectangle {
-                        x: 0.0,
-                        y: 0.0,
-                        width: vp_size.0,
-                        height: vp_size.1,
-                    };
-                    let view_proj = self.tabs[i].scene.camera.borrow().view_proj(bounds);
-                    let all_wires = self.tabs[i].scene.hit_test_wires();
-                    // Mirror the click-selection pick order so the rollover
-                    // highlights every selectable object: wire → hatch →
-                    // block-internal hatch → shaded 3D solid body.
-                    let hovered = scene::pick::hit_test::click_hit(p, &all_wires[..], view_proj, bounds)
-                        .and_then(|s| Scene::handle_from_wire_name(s))
-                        .or_else(|| {
-                            scene::pick::hit_test::click_hit_hatch(
-                                p,
-                                &self.tabs[i].scene.visible_hatches_for_click(),
-                                view_proj,
-                                bounds,
-                            )
-                        })
-                        .or_else(|| {
-                            scene::pick::hit_test::click_hit_insert_hatch(
-                                p,
-                                &self.tabs[i].scene.insert_hatches_for_click(),
-                                view_proj,
-                                bounds,
-                            )
-                        })
-                        .or_else(|| self.tabs[i].scene.mesh_click_hit(p, view_proj, bounds));
-                    self.tabs[i].scene.set_hover_highlight(hovered);
+                    self.tabs[i].scene.set_hover_highlight(None);
+                    self.hover_dwell = Some(super::HoverDwell {
+                        last_move_at: Instant::now(),
+                        point: p,
+                        tile_size: vp_size,
+                        tab: i,
+                    });
                 } else {
                     // Suppress the rollover during a command or a drag.
                     self.tabs[i].scene.set_hover_highlight(None);
+                    self.hover_dwell = None;
                 }
 
                 if self.tabs[i].active_cmd.is_some() {
@@ -4330,6 +4310,60 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::HoverDwellTick => {
+                let Some(dwell) = self.hover_dwell.clone() else {
+                    return Task::none();
+                };
+                if Instant::now()
+                    .duration_since(dwell.last_move_at)
+                    .as_millis()
+                    < super::HOVER_DWELL_MS
+                {
+                    return Task::none();
+                }
+                let i = dwell.tab;
+                // Re-check the gate — drag / command may have started
+                // between the move that armed the dwell and this tick.
+                if i >= self.tabs.len() || self.tabs[i].active_cmd.is_some() {
+                    self.hover_dwell = None;
+                    return Task::none();
+                }
+                let bounds = iced::Rectangle {
+                    x: 0.0,
+                    y: 0.0,
+                    width: dwell.tile_size.0,
+                    height: dwell.tile_size.1,
+                };
+                let p = dwell.point;
+                let view_proj = self.tabs[i].scene.camera.borrow().view_proj(bounds);
+                let all_wires = self.tabs[i].scene.hit_test_wires();
+                // Mirror the click-selection pick order so the rollover
+                // highlights every selectable object: wire → hatch →
+                // block-internal hatch → shaded 3D solid body.
+                let hovered = scene::pick::hit_test::click_hit(p, &all_wires[..], view_proj, bounds)
+                    .and_then(|s| Scene::handle_from_wire_name(s))
+                    .or_else(|| {
+                        scene::pick::hit_test::click_hit_hatch(
+                            p,
+                            &self.tabs[i].scene.visible_hatches_for_click(),
+                            view_proj,
+                            bounds,
+                        )
+                    })
+                    .or_else(|| {
+                        scene::pick::hit_test::click_hit_insert_hatch(
+                            p,
+                            &self.tabs[i].scene.insert_hatches_for_click(),
+                            view_proj,
+                            bounds,
+                        )
+                    })
+                    .or_else(|| self.tabs[i].scene.mesh_click_hit(p, view_proj, bounds));
+                self.tabs[i].scene.set_hover_highlight(hovered);
+                self.hover_dwell = None;
+                Task::none()
+            }
+
             Message::VisibilityPick(idx) => {
                 if let Some(popup) = self.visibility_popup.take() {
                     self.apply_visibility_state(popup.insert_handle, idx);

src/app/view.rs

@@ -1623,6 +1623,14 @@ impl OpenCADStudio {
         } else {
             Subscription::none()
         };
+        // While a rollover pick is queued, drive ticks so it fires the
+        // moment the cursor has been still for the dwell window — without
+        // this `ViewportMove` alone never re-fires once the user stops.
+        let hover_dwell = if self.hover_dwell.is_some() {
+            window::frames().map(|_| Message::HoverDwellTick)
+        } else {
+            Subscription::none()
+        };
         // Blink the MText preview caret while the editor is open.
         let caret_blink = if self.mtext_editor.is_some() {
             iced::time::every(std::time::Duration::from_millis(530))
@@ -1642,6 +1650,7 @@ impl OpenCADStudio {
             frames,
             history_tick,
             grip_dwell,
+            hover_dwell,
             caret_blink,
             web_fonts,
             event::listen_with(|ev, status, win_id| {