The pixel size of the desktop (for want of a better word here) can be larger than the pixel size of the portion that's actually displayed on the physical screen at any one time. I don't know how easy that would be to set up today, but historically, Linux/X11 used that trick to provide more screen real estate than what the physical display was capable of displaying at any one time.

Also, a single window can have multiple, possibly nested, independently scrollable regions. As I am typing this, the smaller comment input field scrolls within the context of the larger web page. There's also technically scrollable regions that we normally don't think of as such; single-line text entry fields are generally horizontally scrollable, but typically don't have any scroll bars.

Whether one counts window chrome into window size or not depends on whether one is wearing window manager glasses or application glasses. Both approaches are valid and have different uses.

There's almost certainly more.

I can't quote all passages because I will exceed the character limit but here is a passage by passage response (sadly, also, Markdown isn't supported in threaded comments):

--- A desktop directory larger just without being able to use scroll bars? I grasp it as some sophisticated 2D or 2.5D behavior (I don't know anything about 3D operating systems).

--- I think you meant to the JavaScript phenomenon of making "collapsed" boxes "uncollapsed".

--- I never tried such glasses so I have no idea what you mean to; I would assume there is some 3D behavior which quite "strips out" the entire 2D or 2.5D situations common with operating systems today