Issue

TLDR; watch the attached video.

Was trying to make a free flowing bottom sheet like view, that allows for the sheet content to also be scrollable.

Was able to create this using .draggable() and a nested scroll connection. Code

Drags are working fine. But flings aren't, when the sheet is still moving. Because the fling velocity being reported in NestedScrollConnection.onPreFling() is wrong. Sometimes even in the opposite direction, if not the wrong magnitude.

Root Cause

I suspect this is because the velocity calculation is based on touch position relative to the component handling the pointer input.

Documentation of PointerInputChange states

position - The Offset of the current pointer event, relative to the containing element

Modifier.draggable which is also used in Modifier.scrollable effectively uses VelocityTracker.addPointerInputChange()

fun VelocityTracker.addPointerInputChange(event: PointerInputChange) {
    @OptIn(ExperimentalComposeUiApi::class)
    event.historical.fastForEach {
        addPosition(it.uptimeMillis, it.position)
    }
    addPosition(event.uptimeMillis, event.position)
}

Source: cs.android.com/VelocityTracker.kt

Potential Fix

If PointerInputChange also had absolutePosition which is relative to the window/screen, and the above function is modified to use absolutePosition instead of position(relative), then we will see the right velocity being calculated when drag has stopped

val event = if (isDragSuccessful) {
  val velocity =
    velocityTracker.calculateVelocity().toFloat(orientation)
  DragStopped(velocity * if (reverseDirection) -1 else 1)
}

Source: cs.android.com/Draggable.kt

I am not sure if we will need relative velocity of the user's fling to the component anywhere. It should be safe to move the implementation VelocityTracker.addPointerInputChange() to use absolutePosition always.

Other components that are affected (currently working around it)

Because of this issue, even Swipeable.kt doesn't use fling velocity to infer the direction of swipe, instead uses the diff of current offset and last offset - Swipeable.kt

Attachments

Code for reproducing the issue: okmanideep/flingvelocityissue
PFA video explaining the issue