Double click on panel divider could make panels sizes equal. Both in Horizontal and Vertical modes. Right click on divider can display menu with percentages of quick resizing of panels like - 25% right panel 75% left panel, 33% 66%, 50/50, 66/33, 75/25 (Horizontal and Vertical modes). Middle click on divider can switch Vertical/Horizontal modes.
I confirm this wish :-) , I remember that Total Commander had at least the first two features, and they came very handy :-).
You can create multiple user actions with different splitter values. This is not the same of course as having a popup menu.