I’m glad that your enjoying the add-on, and I understand your frustration with this issue.
I’ve had this issue myself, and sadly I don’t really know how to fix it!
Whether it shows the top or the bottom of a long menu is up to the internal UI code in Blender (and, after looking for a solution, it seems that there’s no way to set it to always show the top), it depends on where your cursor is (top half of the window: It shows the top of the menu, bottom half of the window: It shows the bottom of the menu).
(If anybody knows if this behavior can be changed, please tell me)
A quick fix would be to add the “Search” option to the quick menu. (Perhaps not the most user friendly way though…)
I, personally, just hit the [Home] key to get to the top.
Only way i can think of to counter this in the add-on, is to add “Pages”, so after say 50 materials, there’s a new “page”/“sub menu” at the bottom with 50 more, etc, etc.
Personally I think that that would be more cumbersome than how it is now. (I also don’t know how feasible that is to implement).
Any suggestions are welcome!