This is a somewhat roundabout way to fix the issue that pressing ^Z
would put the shell prompt in the wrong place (overwriting some of the
status area below the composer). While I'm at it, clean up the suspend
logic and fix some suspend-while-in-alt-screen behavior too.
moves TranscriptApp to be an "overlay", and continue to pump AppEvents
while the transcript is active, but forward all tui handling to the
transcript screen.
