Issue Summary

This happens if "Preferences->Timeline->Drawing" is set to "Keep drawing on the previous keyframe".
On a bitmap layer, if you draw a stroke while the timeline is not on a keyed frame, and then undo it, several odd things happen:

Actual Results

1. A keyframe is generated at the current position in the timeline.
2. The new keyframe is a duplicate of the previous keyframe, but with the stroke undone
3. The previous keyframe retains the stroke that was meant to be undone

It might have other consequences with other tools and layer types, I haven't tried.

Expected Results

1. A new keyframe should not be created.
2. N/A
3. The stroke should be undone on the previous keyframe.

Steps to reproduce

The above should be self-explanatory. If not, let me know and I'll provide a step-by-step.

System Information

• Pencil2D Version: v0.6.5