The scissor rect clips points according to their center, not according to their center + pointSize. This can lead to some strange artifacts during undo/redo or when changing the rotation of the highlighter which causes a re-render of all strokes around the start point.

Possible solutions:

1. don't use scissor test at all, which is what I've done for now to keep rendering correct at the expense of speed, or
2. use quads instead of points, which will raise point memory by 4x, which might be ok