fillMaxHeight

FillHalfHeightModifier

@Composable
fun FillHalfHeightModifier() {
    Box(Modifier.requiredSize(100.dp).background(Color.Red), contentAlignment = Alignment.Center) {
        // The inner Box will be (30.dp x 50.dp).
        Box(Modifier.requiredWidth(30.dp).fillMaxHeight(0.5f).background(color = Color.Magenta))
    }
}

SimpleFillHeightModifier

@Composable
fun SimpleFillHeightModifier() {
    Box(Modifier.fillMaxHeight().background(Color.Red), contentAlignment = Alignment.Center) {
        Box(Modifier.size(100.dp).background(color = Color.Magenta))
    }
}