I got issues with IDE freezing. I undocked scene composer and switched it off. Problems solved. If I need a scene viewer I switch it on in a single window.
Hm, this uses a workaround I put in for when the scene is not running (which should never be the case).. But with the solution @Momoko_Fan is working on the deadlock issues should go away and it will then be able to regularly close the SceneViewer.