I had the same problem on arch linux and linux mint. In both I used Cinnamon interface (GTK3) , so i changed to XFCE4 (GTK2) and KDE (QT) and this bug does not appear anymore. One of these methods may solve your problem: Change your interface (I think you're not going to do this).
Or change the LCL of your project to QT (QT is better than GTK, my opinion).
!!!!
IN EXTREMELY NECESSITY you can change the LCL of your project to GTK3 (GTK3 is beta on lazarus and i don't know if this can solve your problem).
Sorry for my bad english.