With Android 14, apps can opt in for predictive back where they show a preview of what is happening as the system back gesture is happening (rather than only after the user confirms the system back gesture).

The Activity 1.8.0 release adds a new PredictiveBackHandler that allows apps to capture the progress of the system back gesture and use it to drive the state of their own components.

I wanted to use this to start hiding a Material3 Bottom Sheet by writing code like:

PredictiveBackHandler(enabled = showBottomSheet) { progress ->
    try {
        progress.collectLatest {  backEvent ->
            // Progress is 0 (not hidden) to 1 (fully hidden)
            val progress = backEvent.progress
            // TODO: Be able to pass the progress to hide to animate the
            // bottom sheet to a particular gesture controlled progress
            // towards completely hidden
            sheetState.hide(progress)
        }
        // The user has fully committed the system back gesture and the
        // bottom sheet should be completely hidden
        sheetState.hide()
    } catch (e: CancellationException) {
        // The user has cancelled the system back gesture so we should
        // reset its position back to shown
        // TODO: ensure this works even though this suspending scope is cancelled
        sheetState.show()
    }
}

But as per the two TODO messages there, I ran into two issues:

There's no way to drive the progress towards the hidden state from outside the bottom sheet - I see that ModalBottomSheet does this internally to support dragging the bottom sheet down, but this isn't exposed anywhere.
The use of a suspend show method here doesn't actually work well in PredictiveBackHandler since the scope that is provided to the trailing lambda is cancelled by the time we need to call show (similar to other suspending lambdas, it uses coroutine's cancellation as its indicator for cancellation). It would be helpful to have a non-suspending way to set the targetValue and let ModalBottomSheet take care of moving to that state. I think we could possibly work around this by using rememberCoroutineScope and launching the show into that scope, but that seems even harder to get right.