Fix shaky scrolling of LazyColumn when the items are of varying size

[m-sasha]

Proposed Changes

This mostly re-implements commit

793810ba245 25/01/2022, 23:12 Desktop Scrollbar: fix position of a thumb while dragging

which broke smooth content scrolling in LazyColumn when not all items are of the same size.
The problem with that commit was that on each mouse-drag event it did (conceptually)

dragPosition += dragDelta
contentPosition = dragPosition/scrollScale

but when some items are larger than others scrollScale can change significantly between the mouse-drag events, so you could have contentPosition decrease even when dragDelta was always positive.
What we want to do instead is to always add to contentPosition:

contentPosition += dragDelta/scrollScale

The problem that commit was attempting to solve (JetBrains/compose-multiplatform#643) is solved in this commit in a different way:

val sliderDelta = 
    (dragPosition + dragDelta).coerceIn(0f, maxScrollPosition) - 
        dragPosition.coerceIn(0f, maxScrollPosition)
contentPosition += sliderDelta/scrollScale
dragPosition += dragDelta

Note that this trades-off smooth thumb scrolling for smooth content scrolling, as you can't currently have both (because LazyScrollbarAdapter.scrollOffset is discontinuous)

Testing

Test: Manually on the reproducer submitted in the bug report, and also via unit tests.

Note that the existing unit test was faulty in the sense that it passed even in the presence of the behavior the commit was meant to fix. It was faulty in two ways: the mouse events it was generating were missing the scrollbar thumb, and it was scrolling in the wrong direction for the test.

Issues Fixed

Fixes: JetBrains/compose-multiplatform#2337

[m-sasha]

Behavior before:

before.mp4

Behavior after:

after.mp4