logic error in history module when transforming undo/redo against non-user changes

[rhysbrettbowen]

When using history module, if an edit comes in not from the user it goes through the transform. There is an error in the logic where the change that comes in is directly applied to all undos/redos. Issue is that the state between these are different. What it needs to be transformed against is the change after it's been transformed against all previous undos.

Expected behavior:

function quillHistoryTransform(this: any, delta: DeltaStatic) {
  let stackedUndoDelta = delta;
  forEachRight(this.stack.undo, (change: any) => {
    const undo = change.undo;
    change.undo = stackedUndoDelta.transform(undo, true);
    stackedUndoDelta = undo.transform(stackedUndoDelta);
    change.redo = stackedUndoDelta.transform(change.redo, true);
  });
  let stackedRedoDelta = delta;
  forEachRight(this.stack.redo, (change: any) => {
    const redo = change.redo;
    change.redo = stackedUndoDelta.transform(redo, true);
    stackedRedoDelta = redo.transform(stackedRedoDelta);
    change.undo = stackedRedoDelta.transform(change.undo, true);
  });
}

Actual behavior:

    this.stack.undo.forEach(function(change) {
      change.undo = delta.transform(change.undo, true);
      change.redo = delta.transform(change.redo, true);
    });
    this.stack.redo.forEach(function(change) {
      change.undo = delta.transform(change.undo, true);
      change.redo = delta.transform(change.redo, true);
    });

Version:

1.3.6

[rhysbrettbowen]

Written a test to show this - will make a PR to fix:

      this.quill.history.options.delay = 0;
      this.quill.history.options.userOnly = true;
      this.quill.setText('b\n');
      this.quill.insertText(1, 'd', Quill.sources.USER);
      this.quill.insertText(0, 'a', Quill.sources.USER);
      this.quill.insertText(2, 'c', Quill.sources.API);
      expect(this.quill.getText()).toEqual('abcd\n');
      this.quill.history.undo();
      expect(this.quill.getText()).toEqual('bcd\n');
      this.quill.history.undo();
      expect(this.quill.getText()).toEqual('bc\n');
      this.quill.history.redo();
      expect(this.quill.getText()).toEqual('bcd\n');
      this.quill.history.redo();
      expect(this.quill.getText()).toEqual('abcd\n');
    });```