#3101: clear the tab bar's cached rectangles when a control is hidden

Addresses review comment 4453674677: after a frame drew both controls,
turning off the horizontal scroll bar or the sheet tabs left
app.grid.hscroll / app.grid.tab_split holding the old rectangles.
sheet_tabs now clears both before any visibility return.

Test: tab_split::hiding_the_controls_clears_their_cached_rectangles
toggles both options on one live harness.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/tabs.rs

@@ -100,6 +100,9 @@ fn tab_split_handle(app: &mut SheetApp, ui: &mut Ui, split: Rect, bar: (f32, f32
 /// Draws the worksheet tab strip: navigation, scroll/reveal, rename/drag/move with optional copy, context menu, new-sheet button, split handle and grid scrollbar.
 pub fn sheet_tabs(app: &mut SheetApp, ui: &mut Ui) {
     let t = Tokens::get(ui.ctx());
+    // Only the controls drawn this frame have a rectangle: a hidden one keeps no stale geometry.
+    app.grid.hscroll = None;
+    app.grid.tab_split = None;
     // File › Options › Advanced › Display options for this workbook (#3101): with neither the
     // sheet tabs nor the horizontal scroll bar, the bar isn't there at all.
     let view = app.session.active().map(|d| d.wb.view).unwrap_or_default();

crates/ui-egui/tests/it/tab_split.rs

@@ -197,3 +197,24 @@ fn display_options_hide_the_sheet_tabs_and_the_horizontal_scroll_bar() {
     h.run_steps(5);
     assert!(!shows_tab_name(&h) && h.state().grid.hscroll.is_none() && h.state().grid.tab_split.is_none(), "no bar at all");
 }
+
+#[test]
+fn hiding_the_controls_clears_their_cached_rectangles() {
+    // Turned off on a live window, the controls drop the rectangles of the frame that drew them.
+    let mut h = harness(900.0, 1);
+    assert!(h.state().grid.tab_split.is_some() && h.state().grid.hscroll.is_some(), "both controls drawn first");
+
+    h.state_mut().run("view.showSheetTabs", json!({"on": false})).unwrap();
+    h.run_steps(5);
+    assert!(h.state().grid.tab_split.is_none(), "no split handle once the tabs are hidden");
+    assert!(h.state().grid.hscroll.is_some(), "the scroll bar stays");
+
+    h.state_mut().run("view.showSheetTabs", json!({"on": true})).unwrap();
+    h.state_mut().run("view.showHorizontalScrollBar", json!({"on": false})).unwrap();
+    h.run_steps(5);
+    assert!(h.state().grid.hscroll.is_none() && h.state().grid.tab_split.is_none(), "no scroll bar and no split handle");
+
+    h.state_mut().run("view.showSheetTabs", json!({"on": false})).unwrap();
+    h.run_steps(5);
+    assert!(h.state().grid.hscroll.is_none() && h.state().grid.tab_split.is_none(), "no bar at all");
+}