This is quite frustrating. On AE (OSX) for example I am able to scroll through the mask selection drop-down in the layer window, but on Windows the same menue is only navigatable trough key input.
And on a second note: for any drop-down selection that has an index attributed to all entries, this index should be directly adressable. We have layer numbers and can type them in for a drop-down selection. But instead of allowing to type in an exact number, we are only allowed to jump through the first digit. That's just numbing.