Writing Flow: Avoid using state for tracking arrow key navigation

[aduth]

Related: https://github.com/WordPress/gutenberg/pull/2424/files#r142135955

This pull request seeks to refactor the WritingFlow component to eliminate state which could be set on arrow keypresses. Assigning state will incur a rerender, and for the intended usage, we do not need state to be reflected in the render result. Instead, for tracking the "should move" behavior, an instance property is sufficient and avoids any render.

Open questions:

I will need to look back through the old pull request, but at a glance I'm struggling to understand why we need to bind to both key down and key up, particularly because binding to key up causes a laggy feeling to the arrow movement.

Testing instructions:

Verify that there are no regressions in using arrow keys (left, right, up, down) to navigate from one input to the next / previous, e.g. between two paragraph blocks.

[codecov]

Codecov Report

Merging #2894 into master will increase coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2894      +/-   ##
==========================================
+ Coverage   33.92%   33.93%   +0.01%     
==========================================
  Files         190      190              
  Lines        5695     5693       -2     
  Branches      999      999              
==========================================
  Hits         1932     1932              
+ Misses       3183     3181       -2     
  Partials      580      580

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

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[youknowriad]

😅

[youknowriad]

I'm struggling to understand why we need to bind to both key down and key up

some context here #2424 (comment)

Basically, I first tried to only use keyDown which was great until you release the key (keyUp) which was causing the caret to move unexpectedly and I couldn't figure out what was moving the caret on keyup.

So I decided to use keyUp as a workaround but we still need the keyDown because we need to know the caret position before any move happens.

[youknowriad]

🚢 it