test(browser): let dnd-kit see its first resize before text shrinks - #1150

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Summary

• Fixes a WebKit failure on main (e10c4ee, run 38652866838) in the sidebar test that drags one group's header onto another while the text size changes. After the text shrank, the pointer sat over the target group's new header, but the drop line never showed up.
• This is a test timing race. The app behavior is fine. The test now waits for one browser rendering update before it shrinks the text.

Details

• The sidebar uses dnd-kit (the drag-and-drop library) to find which group is under the pointer. dnd-kit measures each group when a drag starts. After that, it measures a group again only when that group's ResizeObserver (a browser callback that fires when an element changes size) reports a resize. It starts these observers at drag start and ignores their first callback, because that callback only reports the starting size.
• The test shrank the text about 10 ms after the drop line first appeared. On a slow WebKit runner, the browser did not render a frame between drag start and the text change. So the starting size and the resize came in one callback. dnd-kit ignored that callback, kept the measurements taken at the large text size, and the pointer never hit a group.
• The test now waits two animation frames before it changes the text. Browsers deliver ResizeObserver callbacks during each rendering update, so the starting-size callback comes first and the text change comes in a separate callback. A real user cannot change the text size within one frame of starting a drag, so this does not hide a product bug.
• Local reproduction: I wrapped ResizeObserver to hold callbacks from drag start through the text change, then sent one callback per observer. That gave the same CI failure (data-drop-edge stayed null at the same assertion). When I sent the held callback before the text change, the test passed.