I don't know about other operating systems, but this is very prolific in Windows.

Let's say you have a scrollable container that items can be dragged onto. You want to drag an item onto the part of the form that isn't currently visible, and since you can't grab the scrollbar whilst your mouse is 'holding' an item, the vast majority of programs will allow you to drag over the top or bottom of the container boundaries and the container will scroll itself up or down respectively.

As a side-point, this action is usually either atrociously slow or impossibly fast. We don't seem to have that one cracked yet, as a whole.

What's odd though is that jiggling the mouse will almost always speed up the scrolling. This is most noticable if the scrollable container is very tall. I personally have taken to making a circular-crank-like motion akin to winding back theatre curtains.

Why is this? Is this an intentional feature of the scroll control, or just an old holdover "bug" behaivour(it's been around for as long as I can remember)? If the former is true, from a UX perspective, has this ever actually been studied and determined to be a good, discoverable feature? I personally have never come across it documented anywhere - I think my own discovery and pick-up of usage just came from noticing it happening by chance.

Note that this isn't referring to this auto-scrolling that speeds up the further away you are from the control boundary. This is fixed speed regardless of distance from the control, that speeds up when you waggle in any direction.

closed as off topic by rk., ChrisF, Matt Obee, JohnGB, Benny Skogberg May 25 '13 at 5:32

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    Maybe they're triggering the scroll event more often because it's happening on every mousemove event, as well as some timed hover event? – Alex Feinman May 24 '13 at 14:08
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    This isn't really a UX question unless you're asking whether it should behave that way. It's a technical implementation question, and as much as I'd love to know why, it's still outside the scope of this site. – JohnGB May 24 '13 at 14:10
  • Would this be better suited to SO, then? I actually wasn't sure whether it fell more onto UX (which it potentially does if its an intentional feature) or implementation side. Seems a bit of both and could even depend on half of the answer – Kai May 24 '13 at 14:15
  • @Kai it probably wouldn't go down well on SO either, it's not the sort of question Stack Exchange is good at answering. (I'm an moderator on SO). – ChrisF May 24 '13 at 14:34