Review fix-up: switch to Home only when the legacy keytip prefix changes

ribbon_keys ran ui.ribbonTab every frame while the prefix was E/O/OC,
forcing the Home tab continuously. Run it once on the prefix change and
add a regression test.

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

crates/ui-egui/src/lib.rs

@@ -519,8 +519,10 @@ impl SheetApp {
             // Leave pointer cancellation to the popup so clicking a menu item still works.
             egui::Popup::close_all(ctx);
         }
-        // The legacy Edit/Format paths lead to the same existing Home menus.
-        if matches!(self.keytips.prefix(), Some("E" | "O" | "OC")) {
+        // The legacy Edit/Format paths lead to the same existing Home menus. Switch once, when
+        // the prefix changes, not every frame (that would reset a tab the user picked meanwhile).
+        let after = self.keytips.prefix();
+        if after != before.as_deref() && matches!(after, Some("E" | "O" | "OC")) {
             self.run_or_alert("ui.ribbonTab", json!({"tab": "Home"}));
             self.ui.ribbon_collapsed = false;
         }

crates/ui-egui/tests/keytips.rs

@@ -208,6 +208,20 @@ fn ribbon_and_legacy_autofit_sequences_resize_the_selected_column() {
     }
 }
 
+#[test]
+fn legacy_prefix_switches_to_home_once_not_every_frame() {
+    let mut h = harness(blank());
+    h.state_mut().ui.ribbon_tab = "Insert".into();
+    sequence(&mut h, &[Key::E], false);
+    assert_eq!(h.state().keytips.prefix(), Some("E"));
+    assert_eq!(h.state().ui.ribbon_tab, "Home");
+    // While the prefix stays the same, later frames must not keep forcing the Home tab.
+    h.state_mut().ui.ribbon_tab = "Insert".into();
+    h.run_steps(3);
+    assert_eq!(h.state().keytips.prefix(), Some("E"));
+    assert_eq!(h.state().ui.ribbon_tab, "Insert");
+}
+
 #[test]
 fn escape_backs_out_without_clearing_copy_and_f10_toggles_off() {
     let mut h = harness(copied_row());