Writing Flow: Fix focus/keyboard issues with multi-select

[mcsf]

Description

#3253 introduced a couple of issues. See:

• Remove focus on multi-select stop #3253 (comment)
• While selecting multiple blocks, the page will crash once you go back to selecting only one block. #3275

This PR reverts the offending commit from #3253, cherry-picks @iseulde's work in #3222, and adds a particular guard against a TypeError reading focus.collapsed.

Testing

• Try multi-selecting blocks with both arrows and pointer.
• Try deleting selections of multiple blocks.
• Try undoing such deletions.
• Try navigating areas of the page with the keyboard, e.g. the toolbar of a block, the focusable elements of the sidebar; try the same when multiple blocks are selected.
• Try selecting multiple blocks with the keyboard, then, with the reverse motion on the keys, coming back to a single-block selection. As of this writing, the ability to keep moving with the keys after returning to a single-block selection is lost. This is likely because d5fdc8c is just a patch that works around the issue, which needs to be properly tackled.

Types of changes

Bug fix.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows has proper inline documentation.

[codecov]

Codecov Report

Merging #3297 into master will increase coverage by 0.07%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #3297      +/-   ##
==========================================
+ Coverage   31.05%   31.13%   +0.07%     
==========================================
  Files         232      232              
  Lines        6521     6524       +3     
  Branches     1163     1164       +1     
==========================================
+ Hits         2025     2031       +6     
+ Misses       3770     3767       -3     
  Partials      726      726

Impacted Files	Coverage Δ
editor/block-settings-menu/index.js	0% <ø> (ø)	⬆️
editor/writing-flow/index.js	0% <ø> (ø)	⬆️
editor/modes/visual-editor/block.js	0% <0%> (ø)	⬆️
components/icon-button/index.js	100% <100%> (ø)	⬆️
components/button/index.js	90.9% <90.9%> (-9.1%)	⬇️

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[ephox-mogran]

As mentioned in a DM (and as @iseulde had considered), we could make the reducer for multi-select switch focus to {} or state.focus (if set) instead of null if the selection start and end were the same. This would mirror how insertBlocks and replaceBlocks, and selectBlock works (focus is initially {}). However, you would lose specific focus things in the block, but maybe that's not important. I think this is an important issue to get fixed (losing your cursor when you condense your selection again), but I'm happy to approve this not crashing fix. which probably should take priority ;)

P.S. It's a shame it doesn't container the other fix in #3280 because that is a bug waiting to happen. But it can be done separately as well.

[ephox-mogran]

@mcsf and @iseulde , it's probably time to revisit this. It is a frustrating user experience that you lose your selection as soon as you condense your multi-block selection back to just one block. It means that you can't multi-select down, change your mind, and then multi-select up.

[mcsf]

@ephox-mogran, totally agree. It's one of my personal pet peeves when using Gutenberg.