@Composable
fun DropdownTest() {

    val (showMenu, setVisibility) = remember { mutableStateOf(true) }
    if (showMenu) {
        Popup(
            alignment = Alignment.TopCenter,
            isFocusable = true,
            onDismissRequest = { setVisibility(false) },
        ) {
            Column(
                modifier = Modifier
                    .fillMaxWidth()
                    .background(Color.Gray)
            ) {
                Text(text = "This should take the whole width, but it takes around 75-80%")
                Text(text = "Is that intended?")
            }
        }
    }
}

setContent