Steps to reproduce:

• Create general empty project
• Select button Navigator
• Click + in Connections to add a new connection
• Select Action "Call Function"
• Select Item "Qt"
• Open menu for Method, then close the menu
• Change Item to "animation"
• Open menu for Method again
• -> background is transparent, see attached picture
• Also happens with other menus in the dialog, e.g. Signal (choose rectangle as target -> open Signal menu and close it, choose button as target and open Signal menu again -> some of the items have a transparent background)