If you need some changes to the composite mode menu, you need to start single mode and copy all of the items. Which is quite a task since there is no easy way to to do it except manually make new entries.
Besides that it’s not easy adding screens at all. The lookup function for screen is not filtering but searching for the first screen with that string in it’s name. Sorting should also be by name alphabetically.
Thank you for sharing your user experience with menu editor.
Improvement for switching main menu to Single mode will become available in Studio 14.0 :
Looks like this is a bug, I’ve created ticket: https://youtrack.cuba-platform.com/issue/STUDIO-8127
This is how standard IntelliJ UI control works. You can press Down arrow and navigate to subsequent usages of the string. Currently we don’t have plans to change those drop-down fields.