"Middle Mouse Roll" on pop-up doesn't trigger the menu

Initial setup... looks like exactly the type of Extension I need....

Followed the instructions, and my mouse functions properly in all applications. Trying the "middle mouse roll" on the corner pop-up does nothing. Tried all combinations of clicking middle mouse, alternative mouse buttons etc. Otherwise the mouse with middle button/scroll works properly in all other applications, including Thunderbird. Is there a "Hot-key" combination to open the context menu perhaps?