Keep a copy of anchor ref to preserve highlighted text selection

[alshakero]

Description

When highlighting a text to change its color, we use document.currentView.getSelection to determine what's selected on the screen. But when you try to type in the color you want using the custom color input, you're also discarding the selection you'd made of the text you want to colorize.

So when useAnchorRef goes to look for the <mark> element within the selected text, it won't find it, since it's actually looking inside the input field.

This PR proposes caching the truthy value of the selection anchor, so user can select other text (like the color string inside the input) without losing track of the text they want to format.

How has this been tested?

Luckily, the issue is reproducible on trunk here. This change demonstrably fixes it.

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

Fixes #36254 and Automattic/wp-calypso#57687

[fullofcaffeine]

This tests well!

[fullofcaffeine]

@jorgefilipecosta @ellatrix I’ve tested that this fixes issue #36254 in v11.8.0 and from this branch (closer to trunk / 11.9.0 RC1), but I’m not sure if this is how you’d prefer to handle this kind of focus/selection related issue long-term. We can follow up with another iteration afterward, if needed.

[fullofcaffeine]

This has been cherry-picked into https://github.com/WordPress/gutenberg/tree/release/11.8.

[youknowriad]

This hook seem a bit misplaced. If we need to be a public API, it should be on the "compose" package (more generic as it has nothing to do with block-editor). But personally, I see it only used once and I'm not sure this hook is very valuable outside that use-case so why not just move it close to its current usage?

[youknowriad]

Also was this cherry-picked to 10.9 as well, since 10.9 is now branched, any cherry-pick made to 10.8 should also be made to 10.9 or the commit won't be on the next major release.

[fullofcaffeine]

@youknowriad jinx! I just did that, and also released RC2 with it https://github.com/WordPress/gutenberg/releases/tag/v11.9.0-rc.2. Thanks for the reminder, anyway! 🙇🏻