Eclipse gets unusable after upgrade to 2018-12.
When opening any kind of file, I constantly get this message popping up.
After searching the web and finding loads of totally stupid suggestions on how to fix this, I stumbled upon this issue, which got me on the right track.
I had XIM configured as input method, which apparently does not work well with Eclipse.
So, either do not use XIM or lauch Eclipse via a script and
unset GTK_IM_MODULE or