I had a similar experience: I built dragging and handle resize, then later added rotation via a bit of trigonometry. Each operation worked fine separately, but if you tried to rotate and then resize, unexpected things happened. I left the bug for years, thinking I needed to come back and just add a bit more trigonometry to get rotated resizing working. When I finally came back to it I spent quite a while hacking at it before realizing that matrices were a much better way to describe the solution.