As we all know, when a user double-clicks on a window title bar, that window gets resized to the full available screen size. At least I have seen this happening in Mac OS. Any idea how to capture this event? What should I listen to in my code so that I know that the user has single or double-clicked the Window title?
Don’t try to intercept interactions with Window decorations directly
Don’t try to work with clicks. Handling state changes due to keyboard or window decoration interactions differs between OSes and isn’t a JavaFX function anyway.
Use Stage properties and API instead
Stage has a maximized property (and iconifed), listen for changes on relevant properties. Similarly, there is a full screen mode which is a bit different (see Stage Java doc).
Creating your own window decorations
If you really want complete control, you can use an undecorated stage style and add your own decorations in JavaFX for handling basic window system functions. That way, a lot of the decoration functionality can be handled internally by your app, but I really don’t recommend that. There was an old project named Undecorator which assisted doing this if you wanted to go that route (I don’t advise coding it yourself).