[JammingBen]

Description

Fixes content that gets lost or changed when a markdown file is opened in the editor and saved.

Fenced code blocks indented 1-3 spaces were dropped by tiptap. They are now parsed as code blocks.

Task lists (markdownTaskList.ts): tiptap counts a tab as one character of indentation and cuts a fixed two characters from every line nested in a task item. That loses the first letter of lines indented by one space and the content of tab indented lines. Leading tabs are now expanded, and lines indented less than the item's content are moved to it. Tabs inside code blocks are kept.

The workarounds can be removed once the referenced tiptap issues are fixed upstream (see linked issues in code).

Related Issue

• Fixes part of text-editor changes the meaning of Markdown on save (empty sub-items, lists with 10+ items, escaped list markers) #3583

How Has This Been Tested?

mkdir md-3616 && cd md-3616
printf -- ' ```\n code\n ```\n' > 01-indented-fence.md
printf -- '  ~~~\n   code\n    x\n  ~~~\n' > 02-indented-tilde-fence.md
printf -- '- a\n ```\n code\n ```\n' > 03-indented-fence-after-list.md
printf -- '- [ ] task\n\n x paragraph\n' > 04-task-paragraph-1-space.md
printf -- '- [ ] a\n b\n c\n' > 05-task-lines-1-space.md
printf -- '- [ ] a\n # h\n' > 06-task-heading-1-space.md
printf -- '- [ ] a\n ```\n code\n   x\n ```\n' > 07-task-code-1-space.md
printf -- '- [ ] a\n  - [ ] b\n\n   x c\n- [ ] d\n' > 08-nested-task-paragraph.md
printf -- '- [ ] a\n\tb\n' > 09-task-line-tab.md
printf -- '- [ ] a\n\t- [ ] b\n' > 10-nested-task-tab.md
printf -- '- [ ] a\n\t```js\n\tconst x = 1\n\t```\n' > 11-task-code-tab.md
printf -- '- [ ] a\n\n\t```\n\tcode\n\t```\n- [ ] b\n\nafter\n' > 12-task-code-tab-blank-line.md

File	main	this PR
01, 02	Code block becomes escaped paragraph text, e.g. \`\`\` code \`\`\`	Code block
03	Code block is dropped, only - a is left	Code block after the list
04	x paragraph becomes paragraph	x paragraph in the task item
05	b and c are dropped	b and c in the task item
06	Heading becomes the text h	Heading h in the task item
07	Code block becomes inline code `ode x`	Code block code / x in the task item
08	x c becomes c	x c in the outer task item
09	b is dropped	b in the task item, with leading spaces (see known limitations)
10	Nested task becomes the text [ ] b	Nested task item b
11	Code block becomes inline code `js onst x = 1`	js code block const x = 1
12	Code block becomes inline code `ode`	Code block code in the first task item

Known limitations

These task list inputs are still not kept as they are. They are no regressions, main handles them the same or worse. They come from tiptap's task list tokenizer (ueberdosis/tiptap#7909) and are left for a follow-up:

• Code in a fence indented less than the fence is lost. - [ ] a\n ```\n x\n ```\n loads as an empty code block, and saving drops the x.
• A task item indented one column past its parent loses its marker. - [ ] a\n - [ ] b\n loads b as the paragraph text [ ] b instead of a task item. The same happens to c in - [ ] a\n - [ ] b\n - [ ] c\n. In CommonMark these items are siblings of their parent.
• A tab indented line below a task item keeps leading spaces. - [ ] a\n\tb\n loads b as the paragraph text b, which is shown shifted to the right and saved as b. On main the line is dropped. fix(editor): keep markdown lists and paragraphs stable across roundtrips #3614 fixes this, it strips the indentation of paragraph lines.