Is it necessary to wait at least 100ms, or is to just necessary to wait any amount of time (i.e. to spin the event loop)? If the latter, `setTimeout(…, 0)` (i.e. zero), which will spin the event loop without waiting any longer than necessary and is the convention in Firefox code when we have to do something after spinning the event loop.

If we do have to wait at least 100ms, however, then that's a problem, because we're probably going to have to wait longer on some machine that is slower than the ones we're testing on. In that case, we should figure out what it is we're actually waiting on and make sure that finishes before we resize the window.