Unpatched easystroke-0.6.0 as in portage will leave behind zombie processes. A patch are available in git, I will attached it here. Please apply. Reproducible: Always
Created attachment 378266 [details, diff] reinstate-signal-handlers-that-got-lost-in-the-gtk3-transition.patch This should prevent zombies.
Created attachment 378268 [details, diff] make-buttons-and-scroll-work-properly-with-send.patch Not sure for what this is, but I have applied it anyway.
Just to be sure: it's not the build system but the program itself, right?
Patches look good. Applied in -r1.
Yes, the program. These two patches are from the next branch: https://github.com/thjaeger/easystroke/commits/next There are one or two more patches available there, you might consider applying them.