With my TeXstudio 2.12.6 (hg 6632:7777b2b8a906) under Windows 8.1, German version, after a while certain pulldown menues cannot be used anymore. E.g., if I click on "Datei", "Idefix" or "Bearbeiten", nothing happens (no menue appears). Other pulldown menues, e.g., "Mathe", still work. I don't remember any action of mine which has led to this behavior. Under "TeXstudio konfigurieren"->"Menü", all menues and menue items are still correctly listed.
I've come across the same effect in QtCreator 4.3.1 (using Qt 5.9.1). This appears to be a Qt issue, so nothing we can do. As it's not reproducible and happens quite rarely, it's even hard to file a meaningful bug report to Qt. (Please report back if you've found a way to reproducibly trigger this behavior).
The recommended workaround is to close TeXstudio and restart it. Then the menu is back to normal.
See also: https://github.com/texstudio-org/texstudio/issues/8