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Sorry, the scroller thing was just an example of when a onPositioned would be call a lot without the size changing.

In this case, how do you expect onSizeChanged modifier behave differently from onPositioned other than performance?

I imagine it would act similarly, but only for the size, and not position.

You can do this yourself with this:

fun Modifier.onSizeChanged(block: (size: IntPxSize) -> Unit): Modifier = object : LayoutModifier2 {
   private var previousSize: IntPxSize = PxSize.Zero

   override Density.modifySize(
     constraints: Constraints,
     layoutDirection: LayoutDirection,
     childSize: IntPxSize
   ) {
     if (childSize != previousSize) {
       previousSize = childSize
       block(childSize)
     }
     return childSize
   }
}

Would you like something that only cares about the content size or is this what you would like added to the library?

Yep, that's exactly what I'd like in the library.

I can also see the benefit of having another modifier to know the content size within the parent too.

Just tried the code above but modifySize doesn't seem to be in LayoutModifier2 (it is in the deprecated LayoutModifier though)

Chris, I'd like to make a sample for using OnSizeChanged. Can you give me a short example of how it would be used?

Here's an example: https://github.com/chrisbanes/tivi/blob/cb/discover-compose/ui-discover/src/main/java/app/tivi/home/discover/Discover.kt#L243

Basically I need to know the height of the app bar, to be able to push down content (y-axis) to be visually below.

Nice! I've put in a simplified version of that.