Summary

Closes #3197.

Dropping a block on the left or right edge of another block (which creates a column list or adds a column) took two undos to revert. The first undo removed the column change but left the dragged block deleted, so the block disappeared until a second undo brought it back. With this change, one undo reverts the whole drop.

Rationale

createMultiColumnHandleDropPlugin's handleDrop first calls editor.removeBlocks(...) to take the dragged blocks out of their old position. It then calls editor.replaceBlocks(...) or editor.updateBlock(...) to build the column list. Each of those calls runs its own editor.transact, so the drop dispatches two transactions.

prosemirror-history merges consecutive transactions into one undo event only when their changed ranges are adjacent. A block dragged from elsewhere in the document usually isn't adjacent to the drop target, so the drop becomes two undo events. Undoing the newer one restores the target and leaves the removal in place.

The reporter's suggestion in the issue (wrapping both steps in editor.transact()) is what this PR does.

Changes

• multiColumnHandleDropPlugin.ts: both branches (adding a column to an existing column list, and creating a new column list) now run the removal and the column-list change inside one editor.transact(() => ...). The nested transact calls in removeBlocks / replaceBlocks / updateBlock reuse the outer transaction, so the drop dispatches once. The logic inside is unchanged apart from indentation.

Impact

• Only the edge-drop path of the multi-column extension is affected. Regular drops are still left to ProseMirror.
• The document after a drop is the same as before. The only difference is that it's one transaction, so onChange listeners see a single change instead of two.

Testing

• New undoing an edge drop cases in src/test/extensions/columnDrop.test.ts, using the existing dropOnEdge helper:

  • drop paragraph-2 on paragraph-1's edge (creates a column list; column-list-0 sits between them)
  • drop paragraph-0 on column-1's edge (adds a column; paragraph-1 sits between them)

  Each test checks that the drop changed the document and that one editor.undo() restores it. The test moves Date.now forward for the drop so it starts its own undo group and isn't merged with the beforeEach document setup.

• On main both cases fail the same way as the issue: after one undo, the dragged block (paragraph-2 / paragraph-0) is missing from the document. With this change both pass.

• Full @blocknote/xl-multi-column suite: 123 passed, 5 skipped. vp lint and vp fmt --check are clean on the touched files.

Screenshots/Video

n/a

Checklist

• Code follows the project's coding standards.
• Unit tests covering the new feature have been added.
• All existing tests pass.
• The documentation has been updated to reflect the new feature (n/a)

Additional Notes

I only ran the jsdom suite locally, not the browser drag-and-drop tests (columnDrop.browser.test.ts).

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