Undo stack loses info after undo followed by new insertion

[andrewheumann]

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Describe the bug
After a sequence where we:

• add some changes
• undo a single change
• add another individual change
  we are no longer able to undo back past the previous undo.

To Reproduce
Here is a test which demonstrates the issue:

/**
 * @param {t.TestCase} tc
 */
export const testUndoAfterChangeAfterUndo = tc => {
  const um = new YMultiDocUndoManager()
  const ydoc1 = new Y.Doc()
  const ytype1 = ydoc1.getText()
  um.addToScope([ytype1])

  ytype1.insert(0, 'a')

  t.assert(ytype1.toString() === 'a')
  t.assert(um.undoStack.length === 1)

  ytype1.insert(1, 'b')

  t.assert(ytype1.toString() === 'ab')
  t.assert(um.undoStack.length === 2)

  ytype1.insert(2, 'c')

  t.assert(ytype1.toString() === 'abc')
  t.assert(um.undoStack.length === 3)

  um.undo()

  t.assert(ytype1.toString() === 'ab')
  t.assert(um.undoStack.length === 2)

  ytype1.insert(2, 'x')

  t.assert(ytype1.toString() === 'abx')
  t.assert(um.undoStack.length === 3) // This is 1, instead of 3

  um.undo()

  t.assert(ytype1.toString() === 'ab')
  t.assert(um.undoStack.length === 2)

  um.undo() // this undo would be unavailable

  t.assert(ytype1.toString() === 'a')
  t.assert(um.undoStack.length === 1)
}

Expected behavior
I expect to be able to continue to undo back through the entire stack after a change that follows an undo.

Screenshots
Environment Information

• NodeJS v16, building this repository from source
• Yjs version 13.5.29

[andrewheumann]

Also worth noting that if I swap out the YMultiDocUndoManager for a Y.UndoManager in the test, it passes.

[andrewheumann]

FWIW this change seems to cause the test to pass, but I confess I'm groping around in the dark a bit here, not sure it's a valid solution:

[andrewheumann]

Thank you for fixing this so quickly! much appreciated!

[dmonad]

Sure thing. Thanks for the test case and the correct fix! I incorporated both in the fix :-)