Description

With the caret right before the text of an ATX heading, Backspace deletes the space after # on screen, but the document is not updated. The line reads #Heading while still drawn as an H1, the document is not marked modified, and File > Save writes # Heading unchanged. The deletion is silently lost; the screen and the saved file disagree.

Steps to Reproduce

1. Create h.md with:

   one
   
   # Heading

2. Open it in MarkText.
3. Click just before the H of Heading (the # marker appears).
4. Press Backspace. The line now reads #Heading. Do not type anything else; one more keystroke in the heading hides the problem.
5. Click at the end of one, then choose File > Save.
6. Choose View > Source Code Mode, or quit MarkText and open h.md again.

The same happens when a list (- one / - two) comes before the heading.

Expected Behavior

The screen, the unsaved marker and the saved file agree. Removing the space turns the line into the paragraph #Heading (which is what happens as soon as one more character is typed), the document is marked modified, and Save writes #Heading.

Actual Behavior

• After Backspace the line reads #Heading, still drawn as a large H1.
• No unsaved marker appears.
• File > Save writes the file unchanged: one, a blank line, # Heading.
• Source Code Mode shows # Heading. Switching back shows the H1 with # Heading again.
• After reopening, the H1 heading Heading is shown; the file still contains # Heading.
• If one more character (for example X) is typed right after the Backspace, that edit is applied: the line becomes the paragraph #XHeading, the unsaved marker appears, and the file saves and reopens as #XHeading.

Screenshots / Logs

No error is shown.

Operating System

macOS

MarkText Version

0.20.0-rc.3 (also reproduced on the current develop branch)

OS Version

macOS 27.0 (arm64)

Reproducibility

• I can reliably reproduce this issue

Possible cause (unverified): AtxHeadingContent.backspaceHandler (packages/muya/src/block/content/atxHeadingContent/index.ts) only converts the heading to a paragraph when the caret is before #. With the caret before H, Format.backspaceHandler lets the browser delete the space natively, and the block text and JSON state are not updated or marked dirty until the next input.