Quote - Ctrl-Shift-B  /  Cmd-Shift-B ?

Thanks a lot; I suspected it might be a RTFM issue, but I didn't locate the Quick Reference Card the manual mentions. Where would that be?
The only PDFs I have is the manual, the Python manual and a file named "freeimage-license"...  :-(

Quote - (edit) on my Mac, there is also a tiny Close window button, top left of the Library screen. But only if it is not docked.

When it's docked you need to click once, wait for the pop up menu, and there select "close". Too slow/tedious if you do this 5 times a minute...