Editor: Reuse the tabbable utility to retrieve the tabbables elements in WritingFlow

[youknowriad]

this reuses the tabbable utility to ensure a more robust arrow navigation.

Testing instructions

• Ensure arrow navigation across blocks is working as intended (same as master)

I'm finding that the button block works better now, it used to be a caret trap but seems to work now.

[codecov]

Codecov Report

Merging #2696 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2696      +/-   ##
==========================================
+ Coverage   33.64%   33.65%   +<.01%     
==========================================
  Files         185      185              
  Lines        5584     5583       -1     
  Branches      973      972       -1     
==========================================
  Hits         1879     1879              
  Misses       3138     3138              
+ Partials      567      566       -1

Impacted Files	Coverage Δ
editor/writing-flow/index.js	0% <0%> (ø)	⬆️

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[aduth]

Could this be simplified to node.contentEditable === 'true' ?

[youknowriad]

sure

[aduth]

Discussed 1-to-1: This seems an improvement, but it's not clear to me the specific behavior we want for arrow navigations, since in some but not all cases we want the block to receive focus (i.e. when there aren't other input fields within). Or at least this is how it appears from the user's perspective. In reality, the block receives focus but then the editable takes over and steals focus to it. I think in a subsequent pull request we might want to consider consolidating this (from Editable to WritingFlow?) and being more explicit about where arrow keys should land focus for the user.