I've always wondered why we've only ever had 2 axes of scrollbars - why not also

z

(and even

t

)

All the scroll bars?

Yes - doesn't always have to be visible, but doesn't that seem like an understandable affordance to zoom out/in and undo/redo?

MS apps often have a zoom scrollbar, Paint for example

I think that scrollbars make the most sense for documents that scroll in 1 dimension. For moving a viewport across a large 2D canvas, I prefer a UI that lets you pan in 2D using mouse movements. Like Google Maps, for example. Using two separate X and Y scrollbars to move around is clumsy by comparison. If I want to visualize where my viewport is in 2D space, I prefer a 2D map of the entire canvas that shows my current viewport position, rather than 2 separate X and Y scroll bars. If you add zooming to this (a Z axis), then again I like the Google Maps UI where you use the mouse scroll wheel or the trackpad scroll gesture to zoom in and out, in preference to a Z axis scroll bar. This is what I've mostly implemented in my Curv project. Curv is missing a "time scrollbar" for rewinding an animation, but I think this is a good idea. It works well in the viewer for Youtube videos.

z and t 'scrollbars' are often implemented by extra arrow buttons pointing left and right, or with a slider control.

Yeah, there are lots of z/t implementation - just seems like it would have been fun to make explicit / all 4-dimensions visible - at least during the visible scrollbar era