Input Interaction: ArrowLeft in multi-line selection prematurely triggers multi-selection

[aduth]

Possibly related: #12322, #14450

Describe the bug

In a paragraph block containing multiple lines of text, a selection spanning multiple lines may prematurely trigger multi-selection when expanding that selection using Shift+ArrowLeft

To reproduce

1. Navigate to Posts > Add New
2. Click the writing prompt
3. Add a paragraph with some text
4. Add a second paragraph with at least two lines of text (does not make a difference whether that occurs naturally or by Shift+Enter line break)
5. Starting in the second line of text, create a selection using Shift+ArrowLeft
6. Continue pressing ArrowLeft while holding Shift
7. Observe that as the selection expands into the first line of text, multi-selection occurs prematurely

Expected behavior

I can expand the selection until reaching the start of the first line of the paragraph. Only at that point should another ArrowLeft expand to create the multi-selection.

Screenshots

Desktop:

• OS: macOS Mojave 10.14.4 (18E226)
• Browser: Chrome Version 73.0.3683.103 (Official Build) (64-bit)
• Version: master (1df3021)

Additional context

The same issue does not appear to occur in the reverse direction, i.e. Shift+ArrowRight does not trigger multi-selection prematurely. However, it never triggers the multi-selection, even when reaching the end of the paragraph, which is similarly an unexpected behavior.

[aduth]

cc @ellatrix

[ellatrix]

@aduth I'll have a look.

[ellatrix]

After looking closer, this actually appears to be a regression caused by #14462, which unfortunately no e2e test caught.