Already present for the tracksticks on Lenovo laptops, missing for mice.
Should be enabled by default if REL_WHEEL is missing, not common in mice anymore but still common in trackballs.
That plays in with Bug 85534, a trackball may not actually have a middle button to press for scrolling, so the solution needs to cover that case.
Also: hold-to-scroll vs. click to enable scroll mode, click to exit mode.
Patchset: 2bbf4a0..d90b75e master -> master