Huh. I wonder why they thought this was such a big deal. I mean... the user caused the problem, and could easily fix it by enlarging the window. Other windowing GUIs handle this just fine.
I'm not sure if it was user tested, but IIRC part of the problem was that there was no visual indication that the menu bar is cut off and some of the commands are inaccessible. We need to remember that this was new - they were introducing a GUI to the masses for the first time ever. Everything hat to be extremely clear.
Good point, and surely a valid concern... one that Apple has forgotten in some places today. Look at the utterly useless icon/thumbnail view in Finder: Contents don't wrap within the window. There could be dozens or hundreds of files off-screen in limbo, and you'll never know.
There are, of course, ways to indicate "more controls this way" with an arrow or other affordance when there's a toolbar or menu overflow, though.
Anyway, the point is that by the time OS X came along, other platforms had solved the problems but Apple rejected those widely-accepted solutions.
When scrollbars are not hidden, icons out of view are more discoverable. This used to be the only way Finder windows worked; there was no option to hide scrollbars. But the Apple decided scrollbars should hide by default, and be very narrow, and be low-contrast… hélas, hélas, c'est pour toujour.
