I've observed some funny behaviour using the scroll wheel (on Mac OSX). To reproduce:

- Open any hyperstack, e.g.
run("Hyperstack...", "channels=2 slices=2 frames=2");

- Open an image or stack, e.g.
run("Blobs (25K)");

- With the last opened image active (focussed), move your mouse over the hyperstack window and move the scroll wheel.

If the currently active window is an image (i.e. not a stack), an error message will be displayed, saying "Stack required".
If the currently active window is a stack, this stack will be scrolled instead of the one below the mouse pointer.

On other (non hyper-) stacks, you can scroll through the stack by mouseover and scroll, without activating the window. I would expect the same behaviour for hyperstacks.

Cheers,
Jan