I already observed this on very slow machines, but now I'm able to catch this in a test when adding flicker coverage for the browser_tabclose_reflows.js test. The current behavior when we close a tab is: 1. Make the closed tab disappear (it becomes fully transparent but its space remains in the tab strip). 2. In another frame, mark another tab as selected. 3. Animate removing the empty area left by the tab that has been closed. Steps 1 and 2 should happen in the same frame.

We already call _blurTab *before* we start the closing tab by removing the fadein attribute. But I guess given the async tab switcher, we don't select the tab synchronously.