The following code has some problem that causes the FAB to be located at the bottom-left of screen when app runs and once a new item is added to the lazy list, it jumps to the bottom-center.

@Composable
fun TodoScreen(
    items: List<TodoItem>,
    onAddItem: (TodoItem) -> Unit,
    onRemoveItem: (TodoItem) -> Unit
) {
    Column {
        LazyColumnFor(
            items = items,
            modifier = Modifier.weight(1f),
            contentPadding = InnerPadding(top = 8.dp)
        ) { todo ->
            TodoRow(
                todo = todo,
                onItemClicked = { onRemoveItem(it) },
                modifier = Modifier.fillParentMaxWidth()
            )
        }

        // For quick testing, a random item generator button
        FloatingActionButton(
                shape = CircleShape,
                elevation = 8.dp,
                icon = {
                    Row(modifier = Modifier.padding(16.dp)){
                        Icon(asset = vectorResource(id = R.drawable.add))
                        Text("Add New Item")
                    }
                         },
                onClick = { onAddItem(generateRandomTodoItem()) },
                modifier = Modifier.gravity(Alignment.CenterHorizontally),
        )
    }
}

Any ideas why this happens? Do I need a constraint mechanism or something for the FAB?

Try replacing the top Colum with a Stack as @ppvi suggested and add this modifier:

Stack(
    modifier = Modifier.fillMaxSize()
) {
 ...  
}

Or even better: a Scaffold

Could you please explain why Scaffold is better for this purpose?

In theory you do not need to use Stack. Just add this Modifier.fillMaxSize to the Column. It happens because the Column is wrap content by default. And when there is no item than the Column is just as width as the FAB

Could you please explain why Scaffold is better for this purpose?

Since you also get the

innerPadding

for your bodyContent. Although I am not quite sure now, if that also accounts the FAB or just the top and bottom bars But apart from that there is also the convenient

floatingActionButtonPosition

parameter there

Also it looks like the LazyColumnFor does not respect the width of the Column. Maybe a bug?