Confirmed in vanilla 6.9 portable. But it's okay in my much customised installed calibre. Pretty sure it's the "Control behaviour of the book list" Tweak.
If "doubleclick_on_library_view" setting is 'open_viewer' (default setting), I get this when I click on a cell on the middle of selected rows:
If "doubleclick_on_library_view" setting is 'do_nothing' (my installed setting), I get this when I click on a cell on the middle of selected rows:
BR