fix(web): ios popup positioning and style

Fixes #5929.
Fixes #6381.

This fixes four separate issues with the longpress popups and key
previews:

1. The key preview was offset by a couple of pixels (unreported issue).
2. The background 'shim' did not cover the key caps, which caused them
   to look startlingly white when the rest of the background greyed
   down (#5929).
3. The popup menu was incorrectly wrapping to two rows due to sizing
   model mismatches (#6381).
4. The popup menu would be realigned incorrectly for the in-app
   keyboard, when moving it to keep it in the keyboard area (#5929).