Keeping heading style when prior block is empty on merging

[bangank36]

What?

Address the issue on #16436 cc: @ellatrix

Why?

• When Pressing backspace on Heading block, it is merged with the empty prior paragraph where its attribute will be replaced with Paragraph attribute, therefore Heading is lost and replaced with regular Paragraph

How?

• Check for the content of the blocks and remove the previous block in case it has empty content
• Assume there are 2 blockA and blockB in the editor, caret is place at start of blockB
  • ➖ is empty text
  • 🆎 is non-empty text

blockA	blockB	Actions
➖	➖	Merge B to A
➖	🆎	Remove A
🆎	➖	Merge B to A
🆎	🆎	Merge B to A

Testing Instructions

• Insert an empty paragraph
• Insert a Heading
• Place caret at the Heading start and press backspace
• Expect the paragraph is removed and the heading is kept with its style

E2E testings

• Introduce new test suite should not merge heading with empty paragraph block on backspace
• Run against npm run test:e2e:playwright -- test/e2e/specs/editor/various/splitting-merging.spec.js
• Test failed on should gracefully handle if placing caret in empty container
  • Reason: the backspace removes all empty paragraphs, making the snapshot mismatch

Testing Instructions for Keyboard

Screenshots or screencast

Kapture.2023-09-04.at.22.50.49.mp4

[ellatrix]

Looking good, just needs some polish :)

[bangank36]

Looking good, just needs some polish :)

Thanks for the thorough review, I made some changes and asked for clarification on #53990 (comment)

[bangank36]

Hi @ellatrix, just a friendly nudge to request your review of the latest changes on my PR when you have a moment. Thanks!

[bangank36]

Closed as resolved in #55134