Updated:

#121786 resolved the original issue in accordance with native behavior, but I am leaving this open to see if folks want the option for the first idea we had for this. If you would like the option to have the scrollbar update after the drag is released, add a 👍

For more context see: #121786 (comment)

Also related: #97676

OP:

Investigating flutter/samples#1680

I suspect (if the scrollbar is being dragged) that the scroll metrics are being updated while drag is active on the scrollbar, leading to the jumpy behavior in the video. The list is not a fixed extent list, so as items are being added, the max scroll extent is changing in real time.

If so, we should defer updating the scrollbar with new metrics until after dragging ends to the user does not get flung around while holding onto the scrollbar.