In PR #20887 the issue got fixed partly, but two issues remained that
unintendedly left room for intermittent failres:
- in the "Ctrl+ArrowLeft/Right" loop the `waitForBrowserTrip` call was
placed _before_ the actual keypresses, which means that for the final
iteration of the loop the last key presses were not properly awaited;
- in the "ArrowLeft/ArrowRight" loop the `waitForBrowserTrip` call was
absent, which means that we didn't wait for key presses at all.
This commit fixes the issues by consistently waiting to a browser trip
_after_ each key press to make sure the sidebar got a chance to render.