Jetpack Compose: Understanding `propagateMinConstraints` in Surface and Box

When working with Jetpack Compose, you might encounter unexpected behavior with the Surface composable, especially when trying to control the size of child elements within it. In this post, we’ll explore a common issue with Surface constraints and how to resolve it.

Problem Description

Imagine you want to create a layout with:

• A full-screen red background
• A small 30dp green box on top of it

To achieve this, you might consider using Surface with a red background color and placing a 30dp green Box inside it. However, instead of the expected layout, you may end up with the entire screen appearing green, with no red background visible.

Expected Result: Full-screen red background with a small 30dp green box.
Actual Result: Entire screen is green.

Analysis of the Issue

The Surface composable in Jetpack Compose has Box as child composable and it has a parameter called propagateMinConstraints, which is set to true by default. 

When this parameter is true, Surface enforces its minimum constraints on its child elements. In other words, it forces its child composables to match its own size constraints, causing the inner Box to expand and fill the entire screen, ignoring the 30.dp size you specified.

This behavior is different from using a regular Box as a container, which doesn’t propagate minimum constraints by default.

Solution

To fix this issue, you have two options:

• Wrap the Surface in a Column or Row: By adding a parent composable (such as Column) around Surface, you avoid the propagateMinConstraints effect on the inner Box.

                Note :  You have to set background color for Column or Row. I've missed in the screenshot below.

• Use Box directly: If you don’t specifically need Surface features, using Box with fillMaxSize() for the background color achieves the desired layout.

Code Examples

Here’s how to implement the solution:

Solution 1: Using Box Without Surface

import androidx.compose.foundation.background
import androidx.compose.foundation.layout.*
import androidx.compose.material3.*
import androidx.compose.runtime.Composable
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp

@Composable
@Preview
fun App() {
    MaterialTheme {
        Box(
            modifier = Modifier
                .fillMaxSize()
                .background(Color.Red)
        ) {
            Box(
                modifier = Modifier
                    .size(30.dp)
                    .background(Color.Green)
            )
        }
    }
}

In this version, we achieve the desired layout without using Surface. The red background covers the screen, and the green Box is displayed at 30dp size.

Solution 2: Wrapping Surface in Column

import androidx.compose.foundation.background
import androidx.compose.foundation.layout.*
import androidx.compose.material3.*
import androidx.compose.runtime.Composable
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp

@Composable
@Preview
fun App() {
    MaterialTheme {
        Column {
            Surface(
                modifier = Modifier
                    .fillMaxSize()
                    .background(Color.Red)
            ) {
                Box(
                    modifier = Modifier
                        .size(30.dp)
                        .background(Color.Green)
                )
            }
        }
    }
}

Here, we wrap Surface in a Column. This avoids the propagateMinConstraints behavior, so the inner Box maintains its 30dp size, and the red background displays correctly.

Conclusion

The default behavior of Surface with propagateMinConstraints = true can sometimes lead to unexpected layout results, especially when nesting fixed-size child elements. By either wrapping Surface in a parent composable (like Column) or using Box directly, you can control the layout more predictably and achieve the desired design.

Hopefully, this guide helps you understand Surface behavior in Jetpack Compose and provides effective solutions for managing constraints in your UI layouts. Happy coding!