Rich Text: Detect handled horizontal navigation by preventDefault

[aduth]

Related: #6712

This pull request seeks to improve upon the implementation of horizontal navigation in RichText from #6712. Per observation in #6712 (comment) , it finds a workaround to TinyMCE's calling of preventDefault on an unhandled caret movement by reassigning its internal event's preventDefault to a noop when the arrow key is moving away from the inline boundary (not within it). This allows WritingFlow to detect the key press as being handled via the standard Event#defaultPrevented, rather than knowledge of an internal _navigationHandled property.

Further, it avoids binding a keydown event to the entire document, which as noted in #6712 (comment) could be problematic.

The revised implementation also seems to correct a potentially wrong behavior where collapsing a selection was wrongly causing the selection to be moved to a previous / next paragraph. This is reflected in updated end-to-end test with explaining comment.

Testing instructions:

Verify that you can traverse into and out of inline boundaries, including those which occur at the beginning or end of a paragraph.

Ensure end-to-end tests pass:

./bin/run-e2e-tests.sh

[tofumatt]

This is so much nicer! The code is easier to understand. Everything still works great for me.

I noticed the splitting of bold text with return seems to work better too–it moved focus to the correct paragraph.

I can't run the e2e tests right now and feel bad blocking this on that.

[aduth]

I noticed the splitting of bold text with return seems to work better too–it moved focus to the correct paragraph.

Nice incidental win 😄 Same issue was also intended to be improved with #7620 .