Description

Triple-clicking a line highlights only that line, but typing a character also consumes the block below it. A thematic break disappears, a list's first item or a quote's text is pulled into the line, and in front matter the body paragraph is pulled into the front matter while the title line is lost. The damage is visible immediately and is written to the saved file. With a table or task list below, the screen changes but the file does not; with a heading below, nothing is selected.

Steps to Reproduce

1. Create t.md with:

   alpha beta
   
   ***

2. Open it in MarkText.
3. Triple-click on the word alpha (three quick clicks without moving the mouse). Only the line alpha beta is highlighted.
4. Type x.
5. Choose File > Save, quit MarkText and open t.md again.

Repeat with each variant. Each file is alpha beta, one blank line, then the "Next block" lines (/ separates lines), except E, whose whole file is given below.

E (triple-click alpha inside the front matter):

---
title: alpha
---

next words

Expected Behavior

Only the highlighted line is replaced by x and the next block is left unchanged (see the "Expected result" column). The screen, the unsaved marker and the saved file agree. Variant I already behaves this way.

Actual Behavior

The next block is merged into the typed line. The unsaved marker appears and the saved file matches the screen:

• A: x*** (rule gone), or x---.
• B: xitem one, then a list with only - item two (ordered: 1. item two).
• C: xquoted words (quote gone). D: xnext words.
• E: --- / xnext words / ---. The title line is gone and the body paragraph is now front matter.

The screen changes but the document does not:

• F: alpha beta is replaced by an empty line with x above the table. G: alpha beta disappears and x is shown inside the task list.
• No unsaved marker appears, File > Save writes the original file, and reopening shows alpha beta again.

Heading below (H): the triple-click highlights nothing, x is inserted before the text, and the file is saved as xalpha beta with the heading unchanged.

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

Related

• [Bug] Cutting a selection that ends in a code block's language box duplicates text and removes the language box #5371 (closed, fixed by fix(muya): keep the code block when a cut ends in its language line (#5371) #5372, not yet released): with a fenced code block with a language (js) below alpha beta, these steps give the same corruption on rc.3 (paragraph xjs, language box removed). That case is left to [Bug] Cutting a selection that ends in a code block's language box duplicates text and removes the language box #5371 and was not rechecked after fix(muya): keep the code block when a cut ends in its language line (#5371) #5372.
• fix(muya): cancel Backspace that turns a setext heading into a paragraph (#5035) #5381 (open PR for Unexpected error with message Failed to execute 'insertBefore' on 'Node': The node before which the new node is to be inserted is not a child of this node. #5035) changes typing over a triple-clicked line above a task list or table (F, G) so the next block stays separate, and lists the merge above an ordinary paragraph as a known gap.
• Extra characters appear when formatting entire line #3591 (closed, old engine): a triple-click selection that included the line break caused extra Bold markers. Related cause, different symptom.

Possible cause (unverified): Chromium ends a triple-click selection at offset 0 of the next block while the highlight covers only the line. The cross-block cut that runs on keydown (packages/muya/src/clipboard/index.ts) then merges what remains of the next block, which is all of it, into the line. When the selection ends on a node with no text (a table, a task checkbox), it cannot be mapped to blocks and the browser edits the DOM alone.