tabs_middle_click_actionsetting with the same options as the already existing
tabs_double_click_actionoption has
Although this would not make sense in current state of affairs - because it is already possible to use single MMB click for closing of a Tab. So it seems that Advanced options are merely missing entry named something like this:
tabs_single_middle_click_actionOr alternatively that that
tabs_double_click_actioncould be transformed into
tabs_closing_clickand expanded to have multiple choices of:
Left Mouse Button Click - Double
Middle Mouse Button Click
Middle Mouse Button Click - Double
But I would suggest the first approach - because in the future, when more features are added to Tabs, each user could decide to what assign mouse buttons when clicking of them is exercised during a hover over a Tab