When entering Compose mode and having “use secondary screen” enabled, the window opens on the main screen and does not fill it out, but only parts of it, showing the underlying programs in the background.
If “use secondary” is unticked, the Compose mode opens on main screen, but the binder is covered, and inaccessible, behind the Compose window on the main screen (“Hide binder” is not ticked) .
It seems that Compose tries to fit to the 2nd screen, but displays it on the main screen.
Drawing the binder to the secondary screen opens Compose there, but as before, the binder is inaccessible and cannot be assigned to the other screen.
Note also that the settings bar on the bottom vanishes after a short time (as it should), but remains inaccessible, so doesn’t turn up again even with the mouse pointer at the bottom, if the Compose doesn’t fill out the whole screen
The workaround is unticking “Use secondary screen”, but then I can’t access the binder on the secondary monitor.
Main: 1600 x 1200
2nd screen: 1280 x1024
OS X 10.11