S5: the gap beside a lagging Chromium page shows the page's color in every motion frame - #36520

One shadow release compile runs at a time and one waits; another dispatch now would evict the waiting one. The scheduled pick compiles the newest feat-cmux-next head when the slot frees, and a PR's fallback compile has its own group.
Source SHA: 534c1532aa0e51d73014dba5f7b2a1f4bf1fd10c