Closes #ISSUE

Additional cleanup and testing for the keystroke input including
- Focused testing of the "previous modifiers" logic in search mode
- Not merging unmodified keystrokes into previous modifier only bindings
(extension of #35208)
- Fixing a bug where input would overflow in search mode when entering
only modifiers
- Additional testing logic to ensure keystrokes updated events are
always emitted correctly

Release Notes:

- N/A *or* Added/Fixed/Improved ...