Focus lost when multi-block selection turn into single-block selection

[mcsf]

Part of umbrella issue #2990.

Issue Overview

Suppose a user has a multi-selection of blocks A, B and C, having started selection at block A. When switching selection back to only encompass block A, focus/selection are seemingly lost. This is most evident and painful when performing keyboard-based navigation and selection, as also voiced in @ephox-mogran's comment #3297 (comment), but is also noticeable when using the pointer, as noted in @iseulde's comment #2990 (comment)

Steps to Reproduce (for bugs)

1. Open a post that contains three Paragraph blocks, each of which with content.
2. Place the caret in the middle of the first block.
3. Hold Shift until the end of the sequence.
4. Start single-block selection by pressing the keyboard's down arrow (↓).
5. Keep pressing ↓ until the second block is reached, thus initiating multi-block selection.
6. Press ↓ once more to reach the third block.
7. Press ↑ twice to switch selection back to the first block only.

Expected Behavior

• Expected to find the caret where selection had been started within the first block, and expected to be able to move it with the arrow keys and continue editing.
• Depending on initial caret position, expected to find some text within the first block to be selected.

Current Behavior

• The caret disappears and seemingly can't be recovered with the arrow keys only.
• Visually, it appears the block itself isn't selected (cf. inspector).
• If dependent on the keyboard, I have to press Tab to outline the block, after which the arrow keys work.
• Depending on initial caret position, some text within the block appears to be selected (highlighted), but it is unreachable.

Possible Solution

Screenshots / Video

Related Issues and/or PRs

Todos

• Tests
• Documentation

[ephox-mogran]

@mcsf, I've got a POC PR (#3584 ) which explores a different way of handling this. Essentially, it allows multi-block selections of only one block, and separates focus from selection. I think it's a reasonable way to keep focus in a sensible location, but I'd like some feedback.

The code itself is not important. It's fairly hacky.

[mcsf]

Testing in 3.2, can no longer repro.