Input Interaction: Shift+ArrowUp uncollapsed single-line selection should expand upward

[aduth]

Steps to Reproduce:

1. Navigate to Posts > Add New
2. Create two paragraphs, each with some amount of text
3. In the first line of the second paragraph, mark a selection using Shift+ArrowRight
   • 
4. Without releasing Shift, press ArrowUp

Expected Result:

Selection should adapt corresponding the expected browser + OS behavior of any other multi-line text field. For example, repeat the steps above as two lines in the "Add Comment" section of this GitHub comment box.

In macOS + Chrome: The selection should collapse.
In all other browsers, OS: The selection should expand upward a line. In terms of blocks, I would expect that given the steps above, it would trigger multi-selection to encompass the two blocks.

It's unclear to me, but it seems that as the outlier, the behavior of macOS + Chrome may be buggy for text fields broadly speaking. In which case, "all other browsers" expected behavior may be what we'd want to seek to emulate.

Actual Result:

In macOS + Chrome: The selection collapses.
In all other browsers, OS: The selection reverts to the beginning of the line.

Noting that this recently seems to have improved (I suspect as of #14453), where previously nothing at all would happen when pressing ArrowUp.

cc @ellatrix

[ellatrix]

Ah, I get it now. :)

[ellatrix]

@aduth Fixed it in #14487