fix(muya): stop drag-selection from expanding to whole blocks - #4477

What

Text drag-selection no longer auto-expands to whole structural blocks. Previously, when a selection's anchor or focus landed inside a code block, list item, or table cell, TextSelection.handleMousemoveOrClick grew the selection to cover the entire enclosing block — snapping the anchor offset to 0 and the focus offset to text.length. The result was that landing in a list item selected the whole item, landing in a code block selected the whole code block, and landing in a table cell selected the whole table.

Expectation (and new behavior): the selection ends exactly where the pointer lands — interior offsets stay interior.

How

• Removed the four expansion branches in TextSelection.handleMousemoveOrClick (and the now-unused direction destructure + Parent / ListItem / TaskListItem imports).
• The isSelectionInSameBlock early return is unchanged.
• This is isolated to the TextSelection text-drag path. It does not affect:
  • TableRectSelection — rectangular cross-cell drag selection (collapses the native range before this code runs, so the same-block early return fires).
  • selectAll() — the Cmd+A escalation lives in a separate facade method.

Testing

• New Chromium e2e regression test (editing/text-selection-no-expand.spec.ts): drags between two list items at interior char offsets and asserts the offsets stay interior (the old expansion forced 0 / text.length). Confirmed red before the fix, green after.
• pnpm -C packages/muya lint:types clean, ESLint clean.
• Full unit suite: 733 passed; check-circular clean.
• Related e2e (editing/ + typing/lists): 21 passed, including table-cell-selection (rectangle selection intact) and selection.spec.ts (Cmd+A across blocks).

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