Hey! Fly-out menus are back!!
I hate those things. Whenever I try to use them, I invariably hover over the category I want, then move my mouse to get to a subcategory item without moving straight to the right first, and the main category menu of course disappears. In other words, instead of doing an inverted L to get to a desired option, I try to take the shortest path, mouse into the space between the category and its subcategory menu, and it instantly disappears. Moving horizontally is an unnatural motion for human hands, which move in arcs. Flyout menus as everybody implements them are therefore not very good from an ergonomic standpoint.
I'm not sure how exactly you'd fix that sort of thing. The only instance of flyout menus I personally know about that didn't do this was the implementation on the original MacOS (not any version of OSX). There you could go from the category to any item in the subcategory menu in a straight line.
Hopefully all of this made sense. I'm surprised how difficult this is to put into words. Here's a picture:
Green is what I try to do and which results in the menu closing. Orange is what you're supposed to do.