I've seen another suggesting you drop first mouse movement event. I'll give that a shot as well.

Edit: scrapped comment re fullscreen.

It seems to general consensus in other forums/platforms is it is SDL/Touchpad issue instead of an application level issue. iOS/Android Dosbox mouse code is, AIUI, unchanged from vanilla. They fixed SDL.

My thoughts yesterday was it was surface resize issue, but I too now think it is relative mouse vs absolute touchpad. Anyway, I'll try again later when I'm at laptop.