With 'popup_dialog' On, a prompt which exactly fills the available
width would start the next line with a space (to separate the prompt
from user's answer) and then have the cursor waiting after it. That's
unlike other behavior in the curses interface where the line split
would be instead of the separating space rather than in addition to it.
Old:
|long prompt?|
| X__________|
New:
|long prompt?|
|X___________|
where the X represents the cursor sitting over the start of blank space
waiting for user's answer.
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