Wait for a standard side sheet to settle before timing its exit in the browser test

The test closed the showcase's standard sheet straight after resizing
the window and sampled its exit, but the sheet has no transitions until
the view settles, two animation frames after Alpine starts, and WebKit
had often not run those frames yet: the sheet closed at once, as it is
meant to while settling, and the test read that as a missing exit. It
failed in Safari when run on its own on 2.1.0's code as well. The
sampling script now waits for `data-md-drawer-settled` first.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/Browser/ContainmentTest.php

@@ -522,11 +522,18 @@ it('gives a closing standard side sheet\'s room back to the content beside it as
 
     // Closed and sampled in one round trip: the sheet's root caught part-way between its width
     // and none, still in the layout, while the column beside it has grown part of the way — not
-    // the whole sheet and its gap handed back in one jump at the end.
+    // the whole sheet and its gap handed back in one jump at the end. Only once the view has
+    // settled, two frames after Alpine starts, which WebKit can still be waiting for here: until
+    // then the sheet has no transitions, on purpose, and closes at once.
     $midExit = $page->script(<<<JS
         (async () => {
             const root = {$root}
             const column = {$column}
+
+            for (let i = 0; i < 100 && ! root.hasAttribute('data-md-drawer-settled'); i++) {
+                await new Promise((resolve) => setTimeout(resolve, 10))
+            }
+
             const open = { root: root.getBoundingClientRect().width, column: column.getBoundingClientRect().width, row: root.parentElement.getBoundingClientRect().width }
             {$toggle}.click()