test: use mutation observer for more reliable simple-combobox tests

Author: tjshiu

GEMINI.md

@@ -0,0 +1,13 @@
+# Jetski Global Instructions & Context
+ 
+ ## Rules
+ - **Skip Tests by Default**: Do not run unit or integration tests automatically after making code changes.
+ - **Run Tests on Demand**: Only run tests when explicitly prompted by the user (e.g., "run tests", "verify with tests").
+ - **Always allowed to run `git status`**: You can run `git status` at any time to check the state of the repository.
+ - **Git Commits**: Follow Angular's commit message format (`<type>(<scope>): <subject>`). Use imperative mood, lowercase subject, and no trailing period. Valid scopes are in `.ng-dev/commit-message.mts`. Scopes can be omitted for `test` or refactoring types. Use `multiple` for cross-component changes. Do not use `aria` as a standalone scope.
+ - **Git Pushing**: Always push branches to the `tjshiu` remote (e.g., `git push tjshiu <branch-name>`), as `origin` is not configured.
+ 
+ ## Aliases
+ - **start-fresh**: Run `git stash push --include-untracked -m "work-in-progress"`, checkout `main`, pull `origin main`, checkout a new branch, and stash pop.
+ - **update-main**: Sync local main with remote using `git checkout main && git pull upstream main && git checkout -`.
+ 

src/aria/private/simple-combobox/simple-combobox.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 
-import {KeyboardEventManager, ClickEventManager, Modifier} from '../behaviors/event-manager';
+import {KeyboardEventManager, ClickEventManager} from '../behaviors/event-manager';
 import {computed, signal, untracked} from '@angular/core';
 import {SignalLike, WritableSignalLike} from '../behaviors/signal-like/signal-like';
 import {ExpansionItem} from '../behaviors/expansion/expansion';