🐛 Bug - Rich text blocks break, duplicate content, and crash when using '=' highlighting

[griffenedge]

Requested by: @griffenedge via YakShaver.ai 🦬
cc: @griffenedge, @isaaclombardssw, @adamcogan, @Marxoz, @PothieuG, @Freego1783

Hi Team!

  🟥  Watch the video (2 min 41 sec)

Pain

In Rules V3 (TinaCMS), rich text blocks behave incorrectly when content containing '=' characters (used for custom highlighting) is added, removed, or edited. The editor shows ghost '=' characters, duplicates entire blocks of content, breaks Markdown parsing (rendering content as code blocks), and can crash with a client-side exception ("Failed to execute 'removeChild' on 'Node'"). This appears to be caused by custom AST manipulation for '=' highlighting interfering with the standard editor flow.

Acceptance Criteria

1. Editing rich text content that includes '=' characters does not duplicate content.
2. Removing or re-adding '=' characters updates the editor state correctly with no ghost characters.
3. Markdown parsing remains valid and does not degrade into unintended code blocks.
4. The editor does not crash when '=' characters are added, removed, or toggled.
5. No client-side exceptions (including removeChild errors) occur during rich text editing.
6. If custom '=' highlighting cannot be made stable, it is disabled until a proper fix is implemented (e.g. in v4 or via TinaCMS).

Reproduce Steps

1. Open Rules V3 (TinaCMS).
2. Navigate to a page with rich text blocks (e.g. SSW Rules).
3. Create a new rich text box.
4. Paste content that includes '=' characters used for highlighting.
5. Remove some '=' characters and then add them back.
6. Observe ghost '=' characters, duplicated content, or broken Markdown rendering.
7. Continue editing until the editor crashes with a client-side exception (removeChild error).

Screenshot

Figure: Equal-sign markup causes duplicated content in rich text blocks

[Aibono1225]

Cc: @tiagov8

As per my conversation with @Marxoz , we will discuss this during the sprint review with @adamcogan