Blocks: Avoid setAttributes on end-of-paragraph split

[aduth]

This pull request is part of a performance audit of the paragraph block. It seeks to resolve an issue where we needlessly call setAttributes on a paragraph block when splitting into a new paragraph, and avoids potentially heavy processing within RichText to determine before and after fragments when it can be inferred by the position of the selection at the end/beginning of the RichText field whether the existing value can be used verbatim; thus also giving opportunity for the paragraph block to do a strict comparison against its own content attribute to determine whether setAttributes needs to be called.

This is one of two setAttributes calls which are currently invoked on the original paragraph when pressing enter at the end of its text. The other is explained at #4956 (comment) .

Testing instructions:

Verify there are no regressions in the behavior of paragraph splitting.

Optional: Validate that UPDATE_BLOCK_ATTRIBUTES is dispatched only once (erroneously, vs. twice on master) when pressing enter at the end of a paragraph block using Redux DevTools Extension.

[youknowriad]

LGTM 👍

[aduth]

I've a few things I'm looking into before merging this:

• Ensuring that there are no problems with calling onSplit directly rather than restoreContentAndSplit. My current conclusion is that we don't need restoreContentAndSplit at all, and it's proposed to be removed in Rich Text: Remove restoreContentAndSplit #7618.
• Ensuring that there's no issue in not reaching the part of splitContent which considers context.paste. I'm still not entirely clear what it's responsible for doing, and am continue to explore further.

[mtias]

This comment was a bit hard to understand.

[aduth]

This comment was a bit hard to understand.

Updated in 2ea9199.

[mtias]

Looks good to me. Thanks for adding comments!

[aduth]

Updates:

• Rebased to resolve conflicts.
• Restored respect of the insertBlocksAfter potentially being undefined for a locked template. See also Block API: Change block mutation functions from undefined to noop in locked template #6587.
• Refactored RichText split once more: This time simplifying to handle edge detection at once, with a single path toward restoreContentAndSplit with handling of paste context. See extended commit message of 7a8ec43.

[jorgefilipecosta]

This pull request is part of a performance audit of the paragraph block. It seeks to resolve an issue where we needlessly call setAttributes on a paragraph block when splitting into a new paragraph.

I think we are stilling making a non required setAttributes call.

I added this paragraph as the content of the post:

<!-- wp:paragraph -->
<p>123456789</p>
<!-- /wp:paragraph -->

I opened redux dev tools in core/editor store.
I put the cursor between 5 and 6 and pressed enter.
I checked that we dispatched UPDATE_BLOCK_ATTRIBUTES with content equal to "123456789" (not needed). Then we inserted a block and then we updated the content of the original block.

It is possible to check that we are doing a non required UPDATE_BLOCK_ATTRIBUTES also by inserting many paragraphs without any content. To undo each paragraph creation we need to use undo two times.

[aduth]

@jorgefilipecosta Nice catch. I can reproduce this. However, I think it's a totally separate issue. Namely, we're causing UPDATE_BLOCK_ATTRIBUTES to be called because even though we're explicitly handling the split on enter press, the event binding for input will cause onChange to be called with the editor content. We should probably prevent this for the enter key (or when otherwise knowing that there is other explicit handling for the input).