According to the comment in line 79, this button should toggle between mixer and editor windows - but it didn't (for me). Apparently that action named has changed in Ardour 5.x - anyway, using this command, it works for me.
git-svn-id: svn://localhost/ardour2/branches/3.0@13002 d708f5d6-7413-0410-9779-e7cbd77b26cf