How align to bottom a row in Jetpack Compose?

You can do it in many ways.

You can use a Column with verticalArrangement = Arrangement.SpaceBetween assigning a weight(1f,false) to the last row:

  Column(
     Modifier.fillMaxHeight(),
     verticalArrangement = Arrangement.SpaceBetween) {
     
     //All elements 
     Column {

        // RED BOX
        
        //...
        Spacer(
            modifier = Modifier
                .fillMaxWidth()
                .background(Green)
                .height(15.dp)
        )

       //... Green box
    }

    //LAST ROW
    Row(
        modifier = Modifier
            .weight(1f, false)
    ) {
        //...
    }
 }

You can use a Spacer(modifier.weight(1f)) between GreenBox and Blue Box to create space between them or you can create your custom column with Layout function and set y position of last Placeable as height of Composable – height of last Composble

    Column(modifier = Modifier
        .height(200.dp)
        .background(Color.LightGray)) {
        Text(
            "First Text",
            modifier = Modifier
                .background(Color(0xffF44336)),
            color = Color.White
        )
        Text(
            "Second Text",
            modifier = Modifier
                .background(Color(0xff9C27B0)),
            color = Color.White
        )
        Spacer(modifier = Modifier.weight(1f))
        Text(
            "Third Text",
            modifier = Modifier
                .background(Color(0xff2196F3)),
            color = Color.White
        )
    }

Result:

Custom Layout

@Composable
private fun CustomColumn(
    modifier: Modifier,
    content: @Composable () -> Unit
) {
    Layout(
        modifier = modifier,
        content = content
    ) { measurables, constraints ->

        val looseConstraints = constraints.copy(
            minWidth = 0,
            maxWidth = constraints.maxWidth,
            minHeight = 0,
            maxHeight = constraints.maxHeight
        )

        // Don't constrain child views further, measure them with given constraints
        // List of measured children
        val placeables = measurables.map { measurable ->
            // Measure each child
            measurable.measure(looseConstraints)
        }

        // Track the y co-ord we have placed children up to
        var yPosition = 0


        // Set the size of the layout as big as it can
        layout(constraints.maxWidth, constraints.maxHeight) {
            // Place children in the parent layout
            placeables.forEachIndexed { index, placeable ->

                println("Placeable width: ${placeable.width}, measuredWidth: ${placeable.measuredWidth}")
                // Position item on the screen
                if (index == placeables.size - 1) {
                    placeable.placeRelative(x = 0, y = constraints.maxHeight - placeable.height)
                } else {
                    placeable.placeRelative(x = 0, y = yPosition)
                }

                // Record the y co-ord placed up to
                yPosition += placeable.height
            }
        }
    }
}

Usage

    CustomColumn(
        modifier = Modifier
            .height(200.dp)
            .background(Color.LightGray)
    ) {
        Text(
            "First Text",
            modifier = Modifier
                .background(Color(0xffF44336)),
            color = Color.White
        )
        Text(
            "Second Text",
            modifier = Modifier
                .background(Color(0xff9C27B0)),
            color = Color.White
        )
        Spacer(modifier = Modifier.weight(1f))
        Text(
            "Third Text",
            modifier = Modifier
                .background(Color(0xff2196F3)),
            color = Color.White
        )
    }

Result:

In this example with Layout you should consider how you measure your measureables with Constraints and your total width and height. It requires a little bit practice but you get more unique designs and with less work(more optimised) composables than ready ones. Here i set layout as maxWidth so no matter which width you assign it takes whole width. It’s for demonstration you can set max width or height in layout based on your needs.