>>>I wouldn't be surprised if you could do that with a horizontal one as well. You have a set of clickable labels, basically, with their surrounding rectangle, laid out in a line. Whether the line is horizontal or vertical affects only their layout, not the functionality. So the decision to have AT clause in activate popup but not in activate menu is actually quite arbitrary. The code inside is the same.
>>>
>>>Or, if it is not the same, then I consider it a serious consequence of the western inductive method of learning (specially sciences). Someone failed to notice the identical behavior and wrote code twice.
>>
>>Obviously the horizontal one is designed to stuck on a window (...), while to vertical one is designed to float and be visible only temporary. For the designers that time it was a huge enough difference?
>
>Or the difference is even older - menus existed even before windowses. They somehow become part of any graphical environment's window handler, so the code for the menu system doesn't come from the language you use, it comes from outside, as a system service (ditto in all flavors of linux I tried so far... and apart from that, I think very few apps I ever saw had their own menu handler, about 99% rely on system). IOW we aren't talking about Dr Dave here, we're talking about the guys who wrote for Mac back in the eighties, or even their predecessors. Who knows what happened then.
>
>I've always felt the menu system as a foreign body grafted on Fox.

I have no idea about real old versions of the fox, but I remember the times in the 80s w/o menus in programs on CP/M, and how it came up and was adopted very fast.
At this time it was state of the art by its simple way to figure out the options of a prog, wasn't it? Remember the printed lists of keyboard short cuts next / below the kbd? It's still an optimum of space used vs possibilities. Even for point'n'click adventures like ribbons or how metro-style is called this days

Sure, they fixed it to the fox, like a lot of stuff is fixed into it.