The presets menu at the foot of the tool rail is a PopupWindow, and
showing a popup takes focus off the develop view until it closes. The
menu held nothing focusable, so Android's Back gesture, pressed to
dismiss it, found no focus item, went unanswered, and the platform
closed the application. Slint closes a popup on Escape by itself but
not on Back.
The menu now holds a key scope, as the film list does, that closes it
on Back or Escape. The next Back leaves develop for the grid.
