The row/column grip and cell-chevron menus are Mantine <Menu>s with
returnFocus:true whose targets live outside the editor's contenteditable. After
a menu action focus returns to that outside target, so ProseMirror's undo keymap
never sees Ctrl+Z until the user clicks back into a cell. Add
refocusEditorAfterMenuClose(editor): on the next frame (after Mantine's
returnFocus) restore editor focus via view.focus(), unless the user intentionally
moved to another input/editable. Wired into both onClose paths (the shared
row/column lifecycle hook + cell-chevron).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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