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I am trying to understand the positioning logic for a scrollbar's thumb after the user clicks on the scrollbar's track. I'm surprised it's not as straight forward as I originally believed.

The expected scenario is what occurs when the distance to move is sufficiently small. The thumb centers itself over the clicked position on the track.

When dealing with larger distances - the thumb does not center itself. Instead, it only moves a fraction of the distance. If the clicking action is held then the scroll continues until the thumb covers the mouse's pointer, but no centering is attempted.

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I'm wondering a few things:

  • Why was the functionality implemented like this? Is this better UX than simply navigating to the location the user indicated?

  • How is the fractional move determined? Is it consistent across scrollbar implementations?

  • As an aside, Option-clicking the scroll bar track on a Mac will scroll to that spot. – Kit Grose May 24 '15 at 14:55
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It was designed for the majority (pareto) use case

With generic scrolled content, it's more typical to read the content page by page than to jump directly to a location.

Therefore, the scroll track was designed as a large click zone for paging down through content, since that was the more common use case.

Note that in practice this is inconsistently implemented, so actual behaviors vary but pagination is the original design intent and UX logic.

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