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Several features in Vanilla Forums appear as a CSS / JS Popup (I think "Lightbox" is the right term?). For example: the "Sign In" function.
It's kinda inconvenient that these popups don't close when clicking outside / in the background. If you clicked something by accident or it's not what you were looking for, you have to explicitly click the 'X', or actually look at the content and click Cancel or Abort or Back etc (if it's there, not always, e.g. 'Preview').
Also, on many other sites, popups like these cause the background to go dark which make them stand out a bit better.
My two suggestions:
Make any popup discardable by clicking anywhere outside
Make the background dim (by a partially transparent black layer) when a popup appears