Quite some time ago I started noticing that the mouse in dosbox was starting to “jump” occasionally. But since I don’t get to play games quite often I ignored and filed it under “possibly driver issues”.
However since this now affects also other parts of the family I dug it out again.
The solution came after realising that the issue was not only related to dosbox but to every SDL application in general. After some probing I found out that the thing that all affected computer had in common (besides running FreeBSD of course ;-)) was that the tool
unclutter ran in each session.
pkill unclutter resolved the issue instantly. However I also removed it from the session scripts to avoid future annoyances.