RichText: add preserveWhiteSpace prop

[ellatrix]

Description

Fixes #18476. White space should be preserved with preformatted <=> paragraph transformations.
Also ensures that any white space in preformatted is preserved. Currently white space is collapsed.

I've done this by adding a preserveWhiteSpace and building it into RichText.

How has this been tested?

Merge a paragraph with a preformatted block.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• 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. .

[SergioEstevao]

Tested it on RN-mobile on iOS and on the playground and all is working correctly now.

Thanks for the fix @ellatrix

[youknowriad]

Why is this in the block.json. Isn't this a RichText prop?

[ellatrix]

It is needed for merging the values, which uses create and toHTMLString under the hood. It is similar to __unstableMultilineWrapperTags: https://github.com/WordPress/gutenberg/search?q=__unstableMultilineWrapperTags&unscoped_q=__unstableMultilineWrapperTags. I marked it unstable as maybe there's other ways we want to do this in future.

[youknowriad]

mmm didn't know about that one. not great to be honest. Maybe the root issue is with the "merge" itself. maybe it should move to RichText somehow.

[ellatrix]

Yes, we should have a look at it. The problem is that selection is stored based on the rich text value and not based on the HTML.

[youknowriad]

Does it mean we should add information about these flags in the selection reducer?

[ellatrix]

I'm not sure... Normally they are set with RichText props. We should document the selection reducers to say that the offsets are based on the rich text values, yes.

[ellatrix]

I've added a bit more documentation for WPBlockSelection. I agree that the merge function could be improved, but this should be looked at separately while this PR fixes an issue. The attribute property has been marked unstable so it could be adjusted in the future.

[ellatrix]

Thanks for the review @SergioEstevao and @youknowriad! Let me know if there's anything I can review for you.