Opening image via context menu of Windows Explorer results in disattached GIMP menu bar
If you open an image in the Windows Explorer via the context menu for a file (i.e., "Edit with GIMP") and then use any of the menu options (e.g., Colours) the menu menu will be separated from the GIMP UI and displayed as an overlay in the Explorer instead. You need to switch back and forth between GIMP and the Explorer once to prevent this. It only happens when GIMP is already open. The problem doesn't occur when GIMP has been launched to edit a particular file via the Explorer's context menu.
Tested on Windows 10 with GIMP 2.10.6.