dispatchers: allow leading whitespace in window parameter

[JManch]

Describe your PR, what does it fix/add?

Fixes #7999

Trims leading whitespace when checking for keywords. Still uses the untrimmed regex if no keyword is provided.

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

No

Is it ready for merging, or does it need work?

Ready