Started happening between 7935.0.0 and 7940.0.0

What steps will reproduce the problem?
1. Use a USB mouse and scroll down one mouse wheel tick.
2. Expect that, as is usual for this mouse, that the page moves 16 pixels.

What is the expected output? What do you see instead?
Instead, page moves 32 pixels.  The difference is visible when comparing how far down a known page the mouse wheel click takes the scroll position.

M49 builds do not behave this way.  Recent M50 builds do.

Problem is found on link, samus, and chell.