fix(ui): reserve scrollbar space and animate event multiselect

Always reserve vertical scrollbar space on base and v5 body
layouts to prevent content shift. Match notification event
multiselect panel open/close transitions to other listboxes.

tests/Unit/BodyScrollbarLayoutTest.php

@@ -0,0 +1,10 @@
+<?php
+
+test('app layouts always reserve space for the vertical scrollbar', function (string $layout) {
+    $contents = file_get_contents(dirname(__DIR__, 2).'/'.$layout);
+
+    expect($contents)->toMatch('/<body(?:\s+[^>]*)?class="[^"]*overflow-y-scroll[^"]*"[^>]*>/');
+})->with([
+    'current interface' => 'resources/views/layouts/base.blade.php',
+    'v5 interface' => 'resources/views/v5/app.blade.php',
+]);