docs(shell): note canvas_region invariant for collapsed sidebar

Recent codex stop-hook iterations exposed three sites where input
hit-test hardcoded LAYER_PANEL_WIDTH while paint followed
canvas_region (which collapses to 0 when sidebar is closed):
over_canvas, apply_wheel cursor offset, toolbar hit rect. Document
the invariant so future widgets don't re-introduce the drift.

crates/CLAUDE.md

@@ -98,9 +98,13 @@ Hit-test runs in REVERSE paint order so the topmost overlay always wins:
 1. TopBar (sidebar toggle button) — also eats other top-bar gaps
 2. AI chat panel (DragHandle starts drag; FocusInput / Send / Example / ToggleCollapse defer to apply_click)
 3. Toolbar (button hits dispatch tools; gaps inside the bounding rect eat clicks)
-4. apply_click → LayerPanel rows / Page rows + chat-defocus
+4. apply_click → LayerPanel rows / Page rows + chat-defocus (skipped when sidebar collapsed)
 5. Empty canvas press → clear `selected` (collapses RightPanel) + start pan-drag
 
+### Coordinate invariant
+
+Every input path that reasons about the canvas region MUST derive its rects from `canvas_region(viewport_w, viewport_h)`. Never reuse `LAYER_PANEL_WIDTH` for hit-test — paint follows `canvas_region`, which collapses to `canvas_left = 0` when `Document.ui.sidebar_open == false`. Sites that proved this rule by violating it: `over_canvas`, `apply_wheel` cursor offset, toolbar hit rect in `apply_press` / `apply_click`. Web `apply_wheel` zoom anchor + `toolbar_rect()` helper follow the same rule.
+
 ## Performance gotchas
 
 - Native chrome paint: ~30 text draws × jian-skia textlayout's per-call `FontCollection::new()` = ~600ms/frame. Fix is the cached typeface path described above. Don't add new draw_text calls without cache awareness.