fix: #4899 keep trailing content out of the child list when outdenting a list item - #4913

Description

Fixes #4899 — and very likely explains #4826/#4827, which were closed as unreproducible. Following the reporter's keystrokes in a Playwright run against the e2e host page reproduces the exact crash deterministically:

Error: Cannot use numerical key for object container
    at assert (ot-json1)
    at insertChildMut → drop → apply
    at JSONState._apply → JSONState._flushOperationCache

Root cause

Instrumenting JSONState._flushOperationCache during the repro shows the state is already schema-invalid one flush before the crash. The Shift+Tab outdent (_unindentListItem, UnindentType.INDENT) produced:

{ "name": "bullet-list", "children": [ { "name": "paragraph", "text": "c" } ] }

— a paragraph as a direct child of a list block, no list-item wrapper.

In packages/muya/src/block/content/paragraphContent/index.ts, the INDENT branch handles two kinds of leftovers when outdenting an item:

• listItem.next — remaining sibling list items, which correctly move into a freshly created nested child list; and
• list.next — blocks that followed the nested list inside the outer item (paragraphs etc.), which are the outdented item's own trailing content — but were appended into that same child list.

ot-json1 doesn't validate the schema, so the corrupt insert applies cleanly; the next edit (the reporter's final Backspace) then produces ops whose composed paths descend into a block object with a numeric array index, which ot-json1 rejects — the deferred flush throws and the renderer reports the crash. That downstream-flush distance is presumably why #4827 was so hard to reproduce.

Fix

Append list.next blocks to the new list item itself, and only create the nested child list when there are sibling list items (listItem.next) to carry. For

- a
  - b
  c

outdenting b now yields c as trailing content of the new - b item (previously: a corrupt list). A side benefit: the outdent no longer leaves an empty bullet-list behind when there are no sibling items.

Tests

• Unit (unindentTrailingContent.spec.ts, boots real Muya in happy-dom like the existing unindentReplacement.spec.ts): outdent with trailing paragraphs, and with both sibling items + trailing paragraphs; asserts every list child is a list item and the markdown keeps c under the outdented item. Both fail without the fix with bullet-list child must be a list item, got paragraph.
• E2E (list-paste-outdent-4899.spec.ts): replays the reporter's exact keystrokes (paste a\n\nb\n\nc into a fresh list, - a line up, Shift+Tab, ↓, Backspace) and asserts no renderer errors and no content loss. Fails without the fix with the exact Cannot use numerical key for object container.

muya unit suite: 1439 passed (1 pre-existing unrelated failure in tableChessboard.spec.ts, fails on clean develop too). muya lint, lint:types, check-circular pass. e2e stability/typing/editing groups: no new failures (2 pre-existing env-dependent failures on this Windows machine, identical on clean develop).

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