Comment 5 for bug 1354213

A possible workaround is explained here:
  https://forums.linuxmint.com/viewtopic.php?t=242806

Adding this:
  GDK_CORE_DEVICE_EVENTS=1
at the end of ~/.profile solves this issue.

GTK3 uses the X Input Device Extension Library by default. It seems that for some reason this extension is the source of the problem.

Setting that environment variable, GKT3 only reacts to core X input events. It doesn't use that extension library.