[MlMt] Hide/Show Preview Pane and keyboard

[Ralph Alvy]

I notice that when I first start a reply to a message, unless I use the 
keyboard first (do some typing), I cannot hide or show the Preview Pane 
via the keyboard shortcut. I must first type some text. This behavior is 
not present when composing a new message. Only when replying.

Not a major problem once you understand how to workaround it (just type 
something first).