Remove unwanted shadow when cards are placed side by side in Jetpack Compose

I've put together a custom shape using two cards that are placed side by side in Jetpack Compose. I want the shape to be elevated, i.e., have a drop shadow. If I elevate the two cards, both cards get shadows as if they're a single shape.

Example code:

import androidx.compose.foundation.layout.PaddingValues
import androidx.compose.foundation.layout.Row
import androidx.compose.foundation.layout.height
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.layout.size
import androidx.compose.foundation.layout.width
import androidx.compose.foundation.shape.RoundedCornerShape
import androidx.compose.material.icons.Icons
import androidx.compose.material.icons.filled.ArrowBackIosNew
import androidx.compose.material3.Button
import androidx.compose.material3.ButtonDefaults
import androidx.compose.material3.Card
import androidx.compose.material3.CardDefaults
import androidx.compose.material3.Icon
import androidx.compose.material3.MaterialTheme
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.RectangleShape
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp
import androidx.compose.ui.zIndex

@Composable
fun MenuExample ()
{
    Row(
        verticalAlignment = Alignment.CenterVertically
    ) {
        Card(
            elevation = CardDefaults.cardElevation(
                defaultElevation = 10.dp
            ),
            modifier = Modifier
                .padding(0.dp)
                .height(80.dp)
                .zIndex(1f)
                .width(40.dp),
            shape = RoundedCornerShape(topStart = 10.dp, bottomStart = 10.dp),
            colors = CardDefaults.cardColors(
                containerColor = MaterialTheme.colorScheme.surface.copy(alpha = 0.98f)
            )
        ) {

            Button(
                onClick = { },
                modifier = Modifier
                    .padding(0.dp)
                    .height(80.dp)
                    .width(40.dp),
                shape = RectangleShape,

                contentPadding = PaddingValues(0.dp),
                colors = ButtonDefaults.buttonColors(
                    containerColor = Color.Transparent,
                    contentColor = Color.Black
                )
            ) {
                Icon(
                    imageVector = Icons.Filled.ArrowBackIosNew,
                    contentDescription = null,
                    modifier = Modifier
                        .size(22.dp)
                        .padding(0.dp)
                )
            }
        }
        Card(
            elevation = CardDefaults.cardElevation(
                defaultElevation = 10.dp,
            ),
            colors = CardDefaults.cardColors(
                containerColor = MaterialTheme.colorScheme.surface.copy(alpha = 0.98f)
            ),
            modifier = Modifier
                .padding(0.dp)
                .width(200.dp)
                .height(200.dp)
                .zIndex(0.5f)
        ) {
        }
    }
}

@Preview
@Composable
fun MenuExamplePreview() {
    MenuExample()
}

I'd like the elevation to treat the two cards as one, meaning that the overlaping area shound not have a drop shadow. How can i remove the unvanded shadow between the two cards (marked with an arrow)?

Solution

I don't know if there is another easier way to achieve this, but one one that i came up with, is to create a custom shape for the card and treating them as a single card.

Make sure to use the latest version of compose to be able to use quadraticTo method

import androidx.compose.foundation.BorderStroke
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.BoxScope
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.ColumnScope
import androidx.compose.foundation.layout.PaddingValues
import androidx.compose.foundation.layout.Row
import androidx.compose.foundation.layout.fillMaxHeight
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.size
import androidx.compose.foundation.layout.width
import androidx.compose.foundation.shape.GenericShape
import androidx.compose.material.icons.Icons
import androidx.compose.material.icons.filled.ArrowBackIosNew
import androidx.compose.material3.Button
import androidx.compose.material3.ButtonDefaults
import androidx.compose.material3.CardColors
import androidx.compose.material3.CardDefaults
import androidx.compose.material3.Icon
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.runtime.Composable
import androidx.compose.runtime.remember
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.geometry.Size
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.RectangleShape
import androidx.compose.ui.platform.LocalDensity
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.Dp
import androidx.compose.ui.unit.DpSize
import androidx.compose.ui.unit.dp


@Composable
fun CustomCard(
    anchorSize : DpSize,
    modifier: Modifier = Modifier,
    roundedCornerSize : Dp = 0.dp,
    elevation: Dp = 0.dp,
    border: BorderStroke? = null,
    colors: CardColors = CardDefaults.cardColors(),
    anchorContent : @Composable BoxScope.() -> Unit = {},
    content : @Composable ColumnScope.() -> Unit = {}
) {
    val anchorSizePx = with(LocalDensity.current) { Size(anchorSize.width.toPx(), anchorSize.height.toPx()) }
    val roundedCornerSizePx = with(LocalDensity.current) { roundedCornerSize.toPx() }

    val customShape = remember {
        GenericShape { size, _ ->
            val width = size.width
            val height = size.height

            val a = (height - anchorSizePx.height) / 2

            moveTo(0f, a + roundedCornerSizePx)

            quadraticTo(0f, a, roundedCornerSizePx, a)
            lineTo(anchorSizePx.width, a)
            lineTo(anchorSizePx.width, roundedCornerSizePx)
            quadraticTo(anchorSizePx.width, 0f, anchorSizePx.width + roundedCornerSizePx, 0f)
            lineTo(width - roundedCornerSizePx, 0f)
            quadraticTo(width, 0f, width, roundedCornerSizePx)
            lineTo(width, height - roundedCornerSizePx)
            quadraticTo(width, height, width - roundedCornerSizePx, height)
            lineTo(anchorSizePx.width + roundedCornerSizePx, height)
            quadraticTo(anchorSizePx.width, height, anchorSizePx.width, height - roundedCornerSizePx)
            lineTo(anchorSizePx.width, height - a)
            lineTo(roundedCornerSizePx, height - a)
            quadraticTo(0f, height - a, 0f, height - a - roundedCornerSizePx)
        }
    }

    Surface(
        modifier = modifier,
        shape = customShape,
        color = colors.containerColor,
        contentColor = colors.contentColor,
        shadowElevation = elevation,
        border = border,
    ) {
        Row{
            Box(
                modifier = Modifier
                    .fillMaxHeight()
                    .width(anchorSize.width),
                contentAlignment = Alignment.Center,
            ) {
                Box(
                    modifier = Modifier
                        .size(anchorSize),
                    content = anchorContent
                )
            }
            Column(content = content)
        }
    }
}


@Preview(showBackground = false)
@Composable
fun CardExamplePreview() {
    CustomCard(
        anchorSize = DpSize(40.dp, 80.dp),
        roundedCornerSize = 10.dp,
        elevation = 10.dp,
        colors = CardDefaults.cardColors(
            containerColor = MaterialTheme.colorScheme.surface.copy(alpha = 0.98f)
        ),
        modifier = Modifier
            .size(200.dp),

        anchorContent = {
            Button(
                onClick = { },
                modifier = Modifier
                    .fillMaxSize(),

                shape = RectangleShape,

                contentPadding = PaddingValues(0.dp),
                colors = ButtonDefaults.buttonColors(
                    containerColor = Color.Transparent,
                    contentColor = Color.Black
                )
            ) {
                Icon(
                    imageVector = Icons.Filled.ArrowBackIosNew,
                    contentDescription = null,
                )
            }
        },
        content = {
            // Panel content goes here
        }
    )
}

Result:

Edit

Another option is to use graphics-shapes library, as pointed by @Kolyneh :

implementation("androidx.graphics:graphics-shapes:1.0.1")

Code:

class CustomShape(
    private val anchorSize : Size,
    private val roundedCornerSize : Float
) : Shape {
    override fun createOutline(
        size: Size,
        layoutDirection: LayoutDirection,
        density: Density
    ): Outline {

        val width = size.width
        val height = size.height

        val anchorWidth = anchorSize.width
        val anchorHeight = anchorSize.height

        val a = (height - anchorHeight) / 2

        val vertices = floatArrayOf(
            0f, a,
            anchorWidth, a,
            anchorWidth, 0f,
            width, 0f,
            width, height,
            anchorWidth, height,
            anchorWidth, height - a,
            0f, height - a
        )

        val rounding = listOf(
            CornerRounding(roundedCornerSize),
            CornerRounding(0f),
            CornerRounding(roundedCornerSize),
            CornerRounding(roundedCornerSize),
            CornerRounding(roundedCornerSize),
            CornerRounding(roundedCornerSize),
            CornerRounding(0f),
            CornerRounding(roundedCornerSize),
        )

        val polygon = RoundedPolygon(
            vertices = vertices,
            perVertexRounding = rounding
        )

        val path = polygon.toPath().asComposePath()

        return Outline.Generic(path)
    }
}

@Composable
fun CustomCard(
    anchorSize : DpSize,
    modifier: Modifier = Modifier,
    roundedCornerSize : Float = 0f,
    elevation: Dp = 0.dp,
    border: BorderStroke? = null,
    colors: CardColors = CardDefaults.cardColors(),
    anchorContent : @Composable BoxScope.() -> Unit = {},
    content : @Composable ColumnScope.() -> Unit = {}
) {
    val anchorSizePx = with(LocalDensity.current) { Size(anchorSize.width.toPx(), anchorSize.height.toPx()) }

    val customShape = remember(anchorSizePx, roundedCornerSize) {
        CustomShape(anchorSizePx, roundedCornerSize)
    }

    Surface(
        modifier = modifier,
        shape = customShape,
        color = colors.containerColor,
        contentColor = colors.contentColor,
        shadowElevation = elevation,
        border = border,
    ) {
        Row{
            Box(
                modifier = Modifier
                    .fillMaxHeight()
                    .width(anchorSize.width),
                contentAlignment = Alignment.Center,
            ) {
                Box(
                    modifier = Modifier
                        .size(anchorSize),
                    content = anchorContent
                )
            }
            Column(content = content)
        }
    }
}


@Preview(showBackground = false)
@Composable
fun CardExamplePreview() {
    CustomCard(
        anchorSize = DpSize(40.dp, 80.dp),
        roundedCornerSize = 20f,
        elevation = 10.dp,
        colors = CardDefaults.cardColors(
            containerColor = MaterialTheme.colorScheme.surface.copy(alpha = 0.98f)
        ),
        modifier = Modifier
            .size(200.dp),

        anchorContent = {
            Button(
                onClick = { },
                modifier = Modifier
                    .fillMaxSize(),

                shape = RectangleShape,

                contentPadding = PaddingValues(0.dp),
                colors = ButtonDefaults.buttonColors(
                    containerColor = Color.Transparent,
                    contentColor = Color.Black
                )
            ) {
                Icon(
                    imageVector = Icons.Filled.ArrowBackIosNew,
                    contentDescription = null,
                )
            }
        },
        content = {
            // Panel content goes here
        }
    )
}