When handling this message, one of the values you receive is the value which expresses "the distanced the wheel is rotated" (or rather, was). I will call it delta1 to avoid confusion. Basically, this works on a scale where a delta1 of [fixed value called WHEEL_DELTA] is equal to [system wide setting] units (lines or whatever). With respect to that fixed value WHEEL_DELTA, it says on MSDN:

"The delta was set to 120 to allow Microsoft or other vendors to build finer-resolution wheels in the future, including perhaps a freely-rotating wheel with no notches. The expectation is that such a device would send more messages per rotation, but with a smaller value in each message. To support this possibility, you should either add the incoming delta values until WHEEL_DELTA is reached (so for a delta-rotation you get the same response), or scroll partial lines in response to the more frequent messages. You could also choose your scroll granularity and accumulate deltas until it is reached."

Unfortunately, many people seem to have glossed over that, and even the rest of the content on the page.

Some of the problems I have faced when using a smooth Microsoft mouse wheel:

  • Applications scrolling in the same direction whichever way you turn the wheel (unless you turn it exceptionally fast). I would guess the bugged code in question would be something like:
    scroll_direction = delta1 >= WHEEL_DELTA ? UP : DOWN
    Microsoft's Performance Analyzer fits this category..
  • Applications not scrolling at all unless you turn the wheel really fast. The problem in this case would be the application ignoring |delta1| values less than WHEEL_DELTA. The new PerfectDisk 10 fits this one.
  • Applications scrolling erratically/"too much". The problem in this case is that they only check the sign of the delta1 value, and ignore the magnitude.
  • Applications lacking common sense with mouse wheel scrolling. For example, on its rotating main menu Windows Media Center generally scrolls several entries at a time on when turning my mouse wheel.
  • Not so much a bug but a nuisance nonetheless: applications accumulating delta1s until |delta1| >= WHEEL_DELTA is reached, when they could in fact have scrolled in smaller increments earlier. This affects many of the lists in the shell on Windows Vista, though I noticed many of those have been changed in Windows 7 beta.

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