Fix focusmanager index corruption

packages/core/src/events/FocusManager.test.ts

@@ -264,4 +264,21 @@ describe('FocusManager Re-entrancy', () => {
         // After re-entrant focusNext, should end up on 'c'
         expect(fm.currentId).toBe('c');
     });
+
+    it('registering a new focusable that sorts before current does not change current focus', () => {
+        const fm = new FocusManager();
+        // A (10), B (20)
+        fm.register(makeWidget('a', 10, true));
+        fm.register(makeWidget('b', 20, true));
+
+        // Focus B
+        fm.focusWidget('b');
+        expect(fm.currentId).toBe('b');
+
+        // Register C with lower tabIndex that sorts before existing items
+        fm.register(makeWidget('c', 5, true));
+
+        // Observable behavior: focused id must remain 'b'
+        expect(fm.currentId).toBe('b');
+    });
 });

packages/core/src/events/FocusManager.ts

@@ -93,9 +93,20 @@ export class FocusManager {
      * they are not lost when App has not yet subscribed to them.
      */
     register(focusable: Focusable): void {
+        // Preserve currently focused id so that sorting the master list
+        // does not accidentally change which widget is focused.
+        const prevFocusedId = this.currentId;
+
         this._focusables.push(focusable);
         this._focusables.sort((a, b) => a.tabIndex - b.tabIndex);
 
+        // If there was a previously focused widget, relocate _currentIndex
+        // to point to the same widget after the sort.
+        if (prevFocusedId) {
+            const newIdx = this._focusables.findIndex(f => f.id === prevFocusedId);
+            if (newIdx >= 0) this._currentIndex = newIdx;
+        }
+
         // Auto-focus the first widget if nothing is focused
         if (this._currentIndex < 0 && focusable.focusable) {
             this._currentIndex = this._focusables.indexOf(focusable);