LazyListState.isScrollInProgress is always false on desktop

[JoeSteven]

Compose-jb version: 1.0.0-beta5
kotlin version: 1.5.31
Hi, I want to save the first item position after scroll, here is my code:

            MaterialTheme {
                val listState = rememberLazyListState()
                LaunchedEffect(Unit) {
                    snapshotFlow { listState.isScrollInProgress }
                        .distinctUntilChanged()
                        .filter {
                            println("is scroll in Progress:$it")
                            !it
                        }.collect {
                            //save state here..
                        }
                }
                LazyColumn(state = listState) {
                    items(500) { index ->
                        Text("Test item $index")
                    }
                }
            }

it works perfect on Android, isScrollInProgress updated every time the list start to scroll and stop scroll

2021-11-19 14:26:37.012 16032-16032/? I/System.out: is scroll in Progress:true
2021-11-19 14:26:37.495 16032-16032/? I/System.out: is scroll in Progress:false
2021-11-19 14:26:37.860 16032-16032/? I/System.out: is scroll in Progress:true
2021-11-19 14:26:39.333 16032-16032/? I/System.out: is scroll in Progress:false
2021-11-19 14:26:58.640 16032-16032/? I/System.out: is scroll in Progress:true
2021-11-19 14:27:00.484 16032-16032/? I/System.out: is scroll in Progress:false

But on desktop (MacOS BigSur/ Windows10) I found that isScrollInProgress is always false even list is clearly in scroll。BTW， I've tested this code both on my project and Compose-jb/examples/imageViewer, the results is the same.

[zacharee]

I'm running into this issue too. It seems like on desktop DefaultScrollableState.scroll() isn't being called. Instead, dispatchRawDelta() is. Since that second method doesn't affect isScrollInProgress, it's always false.

I think it's because of the touchScrollable() vs mouseScroll() differences here.

[SebastianAigner]

Encountered this problem while trying to synchronize the scroll position of multiple parallel LazyColumns.
Inspired by these answers on StackOverflow:

https://stackoverflow.com/questions/71206165/shared-lazyliststate-across-multiple-lazyrow-result-in-strange-behavior?noredirect=1&lq=1,
https://stackoverflow.com/questions/69565303/scroll-multiple-lists-at-different-speeds-in-jetpack-compose/69613111#69613111

Temporarily worked around this by using firstVisibleItemIndex and firstVisibleItemScrollOffset as triggers to perform the scrollToItem:

val idx = lazyListStateA.firstVisibleItemIndex
val off = lazyListStateA.firstVisibleItemScrollOffset

LaunchedEffect(idx, off) {
    lazyListStateB.scrollToItem(idx, off)
}

[MatkovIvan]

It should be mitigated after implementing smooth (animated) scrolling, but if you need to check if there was a scroll, you should subscribe to firstVisibleItemIndex and firstVisibleItemScrollOffset (as @SebastianAigner mentioned above) instead:

snapshotFlow { listState.firstVisibleItemIndex to listState.firstVisibleItemScrollOffset }
    .distinctUntilChanged()

Semantics of isScrollInProgress returns true only if we have a scroll across multiple frames, i.e. we have something continuous, not instant. Unlike dragging where we can detect start, move and end of gesture (in different frames), it's not a case with single event from mouse wheel. As I mentioned above it can be mitigated by introduction animated transaction, but still even after that, there can be a situation where transaction doesn't have an animation, and scroll position will be changed from one place to another in one single frame.

[MatkovIvan]

It worth to mention that smooth (animated) scrolling is merged, so currently there is a TODO in code about this flag. Please note that it's still not exactly about mouse wheel, since it's just single-frame event (as explained above).

[YektaDev]

ScrollState has a similar field. It's also always false on desktop.

[aljohnston112]

It should be mitigated after implementing smooth (animated) scrolling, but if you need to check if there was a scroll, you should subscribe to firstVisibleItemIndex and firstVisibleItemScrollOffset (as @SebastianAigner mentioned above) instead:

snapshotFlow { listState.firstVisibleItemIndex to listState.firstVisibleItemScrollOffset }
    .distinctUntilChanged()

Semantics of isScrollInProgress returns true only if we have a scroll across multiple frames, i.e. we have something continuous, not instant. Unlike dragging where we can detect start, move and end of gesture (in different frames), it's not a case with single event from mouse wheel. As I mentioned above it can be mitigated by introduction animated transaction, but still even after that, there can be a situation where transaction doesn't have an animation, and scroll position will be changed from one place to another in one single frame.

I tried this

    LaunchedEffect(lazyListState) {
        snapshotFlow {
            lazyListState.firstVisibleItemIndex to
                    lazyListState.firstVisibleItemScrollOffset
        }.distinctUntilChanged().collect { (i, offset) ->
            println("Changed")
            lazyListState.scrollToItem(i, offset)
        }
    }

and it only triggers twice and then scrolling stops working.

[MatkovIvan]

@aljohnston112 It's about #3366. Fixed in JetBrains/compose-multiplatform-core#696 It was because conflicting simultaneous animations (triggered by mouse wheel and by your scrollToItem)