IssueTracker

Component used: androidx.paging:paging-compose
Version used: 3.2.1
Devices/Android versions reproduced on: N/A

PageSize.Fixed needs to be marked @Stable (and probably a data class). Currently it will cause recomposition of the Pager if recomposition starts above the Pager and you use PageSize in a way that it gets compared for stability.

The relevant line is here.

Example snippet (you'll need recomposeHighlighter):

@Composable
fun Example() {
    Column {
        var count by remember { mutableIntStateOf(0) }
        Button(onClick = { count++ }) {
            Text("Click me $count")
        }

        // The box wrappers force recomposition to start above the pager
        Box(modifier = Modifier.padding(bottom = count.dp)) {
            // This one's highlighter will not draw, indicating Pager was not recomposed
            Pager(pageSize = PageSize.Fill)
        }

        Box(modifier = Modifier.padding(bottom = count.dp)) {
            // This one's highlighter will increase, indicating Pager was not skipped
            Pager(pageSize = PageSize.Fixed(120.dp))
        }
    }
}

@Composable
fun Pager(pageSize: PageSize) {
    HorizontalPager(
        state = rememberPagerState { 2 },
        pageSize = pageSize,
        modifier = Modifier.recomposeHighlighter()
    ) {
        Text("$it", Modifier.padding(32.dp))
    }
}

In my specific use case, it made sense to remember the PageSize.Fixed, because I was using a dynamic calculation based on the screen width, so that's how I found and worked around this bug.