Give this a try:
When any of the folders have focus, the window title is something like this:
But a Compose Window is:
- Starts out as "New Message"
- When you enter a Subject, the Window title is the Subject
So we need to look for a Window Title that does NOT contain
- Open parenthesis
( - One or more digits
- space
- the text "messages"
This Regex does that:
\(\d+ messages