Scaffold body resize

ComposeCollapsingTopBar/api/ComposeCollapsingTopBar.klib.api

@@ -35,6 +35,10 @@ abstract interface com.flaringapp.compose.topbar.nestedscroll/CollapsingTopBarNe
     open suspend fun (androidx.compose.foundation.gestures/ScrollableState).fling(androidx.compose.foundation.gestures/FlingBehavior, kotlin/Float): kotlin/Float // com.flaringapp.compose.topbar.nestedscroll/CollapsingTopBarNestedScrollHandler.fling|fling@androidx.compose.foundation.gestures.ScrollableState(androidx.compose.foundation.gestures.FlingBehavior;kotlin.Float){}[0]
 }
 
+abstract interface com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffoldBodyScope { // com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffoldBodyScope|null[0]
+    abstract fun (androidx.compose.ui/Modifier).resizeWithCollapse(): androidx.compose.ui/Modifier // com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffoldBodyScope.resizeWithCollapse|resizeWithCollapse@androidx.compose.ui.Modifier(){}[0]
+}
+
 abstract interface com.flaringapp.compose.topbar.snap/CollapsingTopBarSnapBehavior { // com.flaringapp.compose.topbar.snap/CollapsingTopBarSnapBehavior|null[0]
     abstract suspend fun (com.flaringapp.compose.topbar.snap/CollapsingTopBarSnapScope).snap(kotlin/Boolean) // com.flaringapp.compose.topbar.snap/CollapsingTopBarSnapBehavior.snap|snap@com.flaringapp.compose.topbar.snap.CollapsingTopBarSnapScope(kotlin.Boolean){}[0]
 }
@@ -332,7 +336,7 @@ final fun com.flaringapp.compose.topbar.nestedscroll/com_flaringapp_compose_topb
 final fun com.flaringapp.compose.topbar.nestedscroll/com_flaringapp_compose_topbar_nestedscroll_CollapsingTopBarNestedScrollExpand_AtTop$stableprop_getter(): kotlin/Int // com.flaringapp.compose.topbar.nestedscroll/com_flaringapp_compose_topbar_nestedscroll_CollapsingTopBarNestedScrollExpand_AtTop$stableprop_getter|com_flaringapp_compose_topbar_nestedscroll_CollapsingTopBarNestedScrollExpand_AtTop$stableprop_getter(){}[0]
 final fun com.flaringapp.compose.topbar.nestedscroll/com_flaringapp_compose_topbar_nestedscroll_CollapsingTopBarNestedScrollSnap$stableprop_getter(): kotlin/Int // com.flaringapp.compose.topbar.nestedscroll/com_flaringapp_compose_topbar_nestedscroll_CollapsingTopBarNestedScrollSnap$stableprop_getter|com_flaringapp_compose_topbar_nestedscroll_CollapsingTopBarNestedScrollSnap$stableprop_getter(){}[0]
 final fun com.flaringapp.compose.topbar.nestedscroll/com_flaringapp_compose_topbar_nestedscroll_MultiNestedScrollConnection$stableprop_getter(): kotlin/Int // com.flaringapp.compose.topbar.nestedscroll/com_flaringapp_compose_topbar_nestedscroll_MultiNestedScrollConnection$stableprop_getter|com_flaringapp_compose_topbar_nestedscroll_MultiNestedScrollConnection$stableprop_getter(){}[0]
-final fun com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffold(com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffoldScrollMode, androidx.compose.ui/Modifier?, com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffoldState?, kotlin/Boolean, com.flaringapp.compose.topbar.snap/CollapsingTopBarSnapBehavior?, androidx.compose.ui/Modifier?, kotlin/Boolean, kotlin/Function4<com.flaringapp.compose.topbar/CollapsingTopBarScope, com.flaringapp.compose.topbar/CollapsingTopBarState, androidx.compose.runtime/Composer, kotlin/Int, kotlin/Unit>, kotlin/Function2<androidx.compose.runtime/Composer, kotlin/Int, kotlin/Unit>, androidx.compose.runtime/Composer?, kotlin/Int, kotlin/Int) // com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffold|CollapsingTopBarScaffold(com.flaringapp.compose.topbar.scaffold.CollapsingTopBarScaffoldScrollMode;androidx.compose.ui.Modifier?;com.flaringapp.compose.topbar.scaffold.CollapsingTopBarScaffoldState?;kotlin.Boolean;com.flaringapp.compose.topbar.snap.CollapsingTopBarSnapBehavior?;androidx.compose.ui.Modifier?;kotlin.Boolean;kotlin.Function4<com.flaringapp.compose.topbar.CollapsingTopBarScope,com.flaringapp.compose.topbar.CollapsingTopBarState,androidx.compose.runtime.Composer,kotlin.Int,kotlin.Unit>;kotlin.Function2<androidx.compose.runtime.Composer,kotlin.Int,kotlin.Unit>;androidx.compose.runtime.Composer?;kotlin.Int;kotlin.Int){}[0]
+final fun com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffold(com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffoldScrollMode, androidx.compose.ui/Modifier?, com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffoldState?, kotlin/Boolean, com.flaringapp.compose.topbar.snap/CollapsingTopBarSnapBehavior?, androidx.compose.ui/Modifier?, kotlin/Boolean, kotlin/Function4<com.flaringapp.compose.topbar/CollapsingTopBarScope, com.flaringapp.compose.topbar/CollapsingTopBarState, androidx.compose.runtime/Composer, kotlin/Int, kotlin/Unit>, kotlin/Function3<com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffoldBodyScope, androidx.compose.runtime/Composer, kotlin/Int, kotlin/Unit>, androidx.compose.runtime/Composer?, kotlin/Int, kotlin/Int) // com.flaringapp.compose.topbar.scaffold/CollapsingTopBarScaffold|CollapsingTopBarScaffold(com.flaringapp.compose.topbar.scaffold.CollapsingTopBarScaffoldScrollMode;androidx.compose.ui.Modifier?;com.flaringapp.compose.topbar.scaffold.CollapsingTopBarScaffoldState?;kotlin.Boolean;com.flaringapp.compose.topbar.snap.CollapsingTopBarSnapBehavior?;androidx.compose.ui.Modifier?;kotlin.Boolean;kotlin.Function4<com.flaringapp.compose.topbar.CollapsingTopBarScope,com.flaringapp.compose.topbar.CollapsingTopBarState,androidx.compose.runtime.Composer,kotlin.Int,kotlin.Unit>;kotlin.Function3<com.flaringapp.compose.topbar.scaffold.CollapsingTopBarScaffoldBodyScope,androidx.compose.runtime.Composer,kotlin.Int,kotlin.Unit>;androidx.compose.runtime.Composer?;kotlin.Int;kotlin.Int){}[0]
 final fun com.flaringapp.compose.topbar.scaffold/com_flaringapp_compose_topbar_scaffold_CollapsingTopBarScaffoldScrollMode$stableprop_getter(): kotlin/Int // com.flaringapp.compose.topbar.scaffold/com_flaringapp_compose_topbar_scaffold_CollapsingTopBarScaffoldScrollMode$stableprop_getter|com_flaringapp_compose_topbar_scaffold_CollapsingTopBarScaffoldScrollMode$stableprop_getter(){}[0]
 final fun com.flaringapp.compose.topbar.scaffold/com_flaringapp_compose_topbar_scaffold_CollapsingTopBarScaffoldScrollMode_Exit$stableprop_getter(): kotlin/Int // com.flaringapp.compose.topbar.scaffold/com_flaringapp_compose_topbar_scaffold_CollapsingTopBarScaffoldScrollMode_Exit$stableprop_getter|com_flaringapp_compose_topbar_scaffold_CollapsingTopBarScaffoldScrollMode_Exit$stableprop_getter(){}[0]
 final fun com.flaringapp.compose.topbar.scaffold/com_flaringapp_compose_topbar_scaffold_CollapsingTopBarScaffoldState$stableprop_getter(): kotlin/Int // com.flaringapp.compose.topbar.scaffold/com_flaringapp_compose_topbar_scaffold_CollapsingTopBarScaffoldState$stableprop_getter|com_flaringapp_compose_topbar_scaffold_CollapsingTopBarScaffoldState$stableprop_getter(){}[0]

ComposeCollapsingTopBar/src/commonMain/kotlin/com/flaringapp/compose/topbar/scaffold/CollapsingTopBarScaffold.kt

@@ -16,8 +16,10 @@
 
 package com.flaringapp.compose.topbar.scaffold
 
+import androidx.compose.foundation.layout.LayoutScopeMarker
 import androidx.compose.foundation.layout.offset
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.Immutable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
@@ -26,6 +28,9 @@ import androidx.compose.runtime.remember
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.input.nestedscroll.nestedScroll
 import androidx.compose.ui.layout.Layout
+import androidx.compose.ui.layout.Measurable
+import androidx.compose.ui.layout.ParentDataModifier
+import androidx.compose.ui.unit.Density
 import androidx.compose.ui.unit.IntOffset
 import androidx.compose.ui.unit.constrainHeight
 import androidx.compose.ui.unit.constrainWidth
@@ -63,9 +68,13 @@ import kotlin.math.max
  * @param topBarClipToBounds the flag whether or not to automatically clip top bar content [topBar]
  * to the actual collapse height.
  * @param topBar the content of [CollapsingTopBar].
- * @param body the scrollable content under collapsing top bar. If you don't use any by default
- * scrollable container (e.g. [androidx.compose.foundation.lazy.LazyList]), then make sure to
- * apply custom [androidx.compose.foundation.verticalScroll] modifier.
+ * @param body the content under collapsing top bar. By default, direct body children are measured
+ * against collapsed top bar height to minimize remeasurement during collapse. Use
+ * [CollapsingTopBarScaffoldBodyScope.resizeWithCollapse] only for direct body children that must
+ * resize with the current top bar height instead, such as lightweight overlays. It may increase
+ * remeasurement cost and is generally not recommended for scrollable content. If you don't use any
+ * by default scrollable container (e.g. [androidx.compose.foundation.lazy.LazyList]), then make
+ * sure to apply custom [androidx.compose.foundation.verticalScroll] modifier.
  *
  * @see CollapsingTopBar
  */
@@ -80,7 +89,7 @@ public fun CollapsingTopBarScaffold(
     topBarModifier: Modifier = Modifier,
     topBarClipToBounds: Boolean = true,
     topBar: @Composable CollapsingTopBarScope.(topBarState: CollapsingTopBarState) -> Unit,
-    body: @Composable () -> Unit,
+    body: @Composable CollapsingTopBarScaffoldBodyScope.() -> Unit,
 ) {
     LaunchedEffect(scrollMode.canExit) {
         if (!scrollMode.canExit) {
@@ -125,7 +134,7 @@ public fun CollapsingTopBarScaffold(
                 topBar(state.topBarState)
             }
 
-            body()
+            CollapsingTopBarScaffoldBodyScopeInstance.body()
         },
         modifier = modifier
             .then(
@@ -140,15 +149,26 @@ public fun CollapsingTopBarScaffold(
         val topBarPlaceable = measurables[0].measure(topBarConstraints)
 
         val bodyPlaceables = if (measurables.size > 1) {
-            val bodyHeight = (constraints.maxHeight - topBarMinHeightState).coerceAtLeast(0)
-
-            val bodyConstraints = constraints.copy(
+            val collapsedBodyConstrains = constraints.copy(
                 minWidth = 0,
                 minHeight = 0,
-                maxHeight = bodyHeight,
+                maxHeight = (constraints.maxHeight - topBarMinHeightState).coerceAtLeast(0),
+            )
+            val resizedBodyConstrains = collapsedBodyConstrains.copy(
+                maxHeight = (constraints.maxHeight - state.totalTopBarHeight.toInt())
+                    .coerceAtLeast(0),
             )
+
             val bodyMeasurables = measurables.subList(1, measurables.size)
-            bodyMeasurables.map { it.measure(bodyConstraints) }
+            bodyMeasurables.map { measurable ->
+                val nodeConstraints = if (measurable.bodyParentData?.resizeWithCollapse == true) {
+                    resizedBodyConstrains
+                } else {
+                    collapsedBodyConstrains
+                }
+
+                measurable.measure(nodeConstraints)
+            }
         } else {
             emptyList()
         }
@@ -175,3 +195,53 @@ public fun CollapsingTopBarScaffold(
         }
     }
 }
+
+/**
+ * Scope for direct body children of [CollapsingTopBarScaffold].
+ */
+@LayoutScopeMarker
+@Immutable
+public interface CollapsingTopBarScaffoldBodyScope {
+
+    /**
+     * Opt this direct scaffold body child into measurement against current visible top bar height
+     * instead of collapsed top bar height.
+     *
+     * This may increase remeasurement cost during collapse. Prefer the default scaffold body
+     * behavior unless this child must respond to the current visible top bar height, e.g. for a
+     * lightweight overlay.
+     */
+    public fun Modifier.resizeWithCollapse(): Modifier
+}
+
+private object CollapsingTopBarScaffoldBodyScopeInstance : CollapsingTopBarScaffoldBodyScope {
+
+    override fun Modifier.resizeWithCollapse(): Modifier {
+        return then(ResizeWithCollapseModifier())
+    }
+}
+
+private class ResizeWithCollapseModifier : CollapsingTopBarScaffoldBodyParentDataModifier() {
+    override fun modifyParentData(parentData: CollapsingTopBarScaffoldBodyParentData) {
+        parentData.resizeWithCollapse = true
+    }
+}
+
+private abstract class CollapsingTopBarScaffoldBodyParentDataModifier : ParentDataModifier {
+
+    override fun Density.modifyParentData(parentData: Any?): Any {
+        val data = parentData as? CollapsingTopBarScaffoldBodyParentData
+            ?: CollapsingTopBarScaffoldBodyParentData()
+        this@CollapsingTopBarScaffoldBodyParentDataModifier.modifyParentData(data)
+        return data
+    }
+
+    protected abstract fun modifyParentData(parentData: CollapsingTopBarScaffoldBodyParentData)
+}
+
+private data class CollapsingTopBarScaffoldBodyParentData(
+    var resizeWithCollapse: Boolean = false,
+)
+
+private val Measurable.bodyParentData: CollapsingTopBarScaffoldBodyParentData?
+    get() = parentData as? CollapsingTopBarScaffoldBodyParentData

README.md

@@ -45,9 +45,10 @@ dependencyResolutionManagement {
 
 ## Usage
 
-You need a scrollable content for the collapsing header to work. For the sake of optimization, this
-library does something similar to `CoordinatorLayout`: instead of resizing a content as a top bar
-collapses, the content is simply offset downward. That's why you need a `Scaffold`-like wrapper.
+You need scrollable content for the collapsing header to work. For the sake of optimization, this
+library does something similar to `CoordinatorLayout`: by default, body content is measured as if
+the top bar were already collapsed, and then simply offset as the top bar expands and
+collapses. That's why you need a `Scaffold`-like wrapper.
 
 ```kotlin
 CollapsingTopBarScaffold(
@@ -63,6 +64,31 @@ CollapsingTopBarScaffold(
 )
 ```
 
+In some cases you may want a direct body child to resize together with collapse instead of using
+the default fixed measurement strategy. For those cases, `CollapsingTopBarScaffold()` exposes
+`CollapsingTopBarScaffoldBodyScope.resizeWithCollapse()`:
+
+```kotlin
+CollapsingTopBarScaffold(
+    scrollMode = CollapsingTopBarScaffoldScrollMode.collapse(expandAlways = false),
+    topBar = {
+        TopBarImage()
+        TopAppBar()
+    },
+    body = {
+        ScrollableContent()
+        Overlay(
+            modifier = Modifier.resizeWithCollapse(),
+        )
+    },
+)
+```
+
+Use `resizeWithCollapse()` as a targeted override, typically for lightweight direct children such
+as overlays or panels that must track the currently visible body height. It may increase
+remeasurement cost during collapse, so the default scaffold behavior remains the preferred choice
+for scrollable content.
+
 `CollapsingTopBarScaffold()` is flexible enough to cover a variety of use cases. However, there's
 some extra room for customization: you can use `CollapsingTopBar()` as the actual collapsing header,
 and implement your own mechanism to control header and content positioning. Feel free to take a

sample/shared/src/commonMain/kotlin/com/flaringapp/compose/topbar/sample/shared/ui/samples/CollapsingTopBarSampleGroups.kt

@@ -18,6 +18,7 @@ package com.flaringapp.compose.topbar.sample.shared.ui.samples
 
 import com.flaringapp.compose.topbar.sample.shared.ui.samples.advanced.AppBarScrimSample
 import com.flaringapp.compose.topbar.sample.shared.ui.samples.advanced.AppBarShadowSample
+import com.flaringapp.compose.topbar.sample.shared.ui.samples.advanced.BodyResizeSample
 import com.flaringapp.compose.topbar.sample.shared.ui.samples.advanced.FloatingElementSample
 import com.flaringapp.compose.topbar.sample.shared.ui.samples.advanced.ManualCollapsingControlsSample
 import com.flaringapp.compose.topbar.sample.shared.ui.samples.advanced.ParallaxCollapsingSample
@@ -67,6 +68,7 @@ object CollapsingTopBarSampleGroups {
             AppBarShadowSample,
             AppBarScrimSample,
             ManualCollapsingControlsSample,
+            BodyResizeSample,
             FloatingElementSample,
         )
 

sample/shared/src/commonMain/kotlin/com/flaringapp/compose/topbar/sample/shared/ui/samples/advanced/BodyResizeSample.kt

@@ -0,0 +1,127 @@
+/*
+ * Copyright 2026 Flaringapp
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.flaringapp.compose.topbar.sample.shared.ui.samples.advanced
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Brush
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.tooling.preview.Preview
+import com.flaringapp.compose.topbar.sample.shared.ui.samples.CollapsingTopBarSample
+import com.flaringapp.compose.topbar.sample.shared.ui.samples.CollapsingTopBarSampleDogDefaults
+import com.flaringapp.compose.topbar.sample.shared.ui.samples.common.SampleTopAppBar
+import com.flaringapp.compose.topbar.sample.shared.ui.samples.common.SampleTopBarImage
+import com.flaringapp.compose.topbar.sample.shared.ui.theme.ComposeCollapsingTopBarTheme
+import com.flaringapp.compose.topbar.scaffold.CollapsingTopBarScaffold
+import com.flaringapp.compose.topbar.scaffold.CollapsingTopBarScaffoldScrollMode
+
+object BodyResizeSample : CollapsingTopBarSample {
+
+    override val name: String = "Body Resize"
+
+    @Composable
+    override fun Content(onBack: () -> Unit) {
+        BodyResizeSampleContent(onBack = onBack)
+    }
+}
+
+@Composable
+fun BodyResizeSampleContent(
+    onBack: () -> Unit,
+    modifier: Modifier = Modifier,
+) {
+    Surface(
+        modifier = modifier.fillMaxSize(),
+    ) {
+        CollapsingContent(
+            onBack = onBack,
+        )
+    }
+}
+
+@Composable
+private fun CollapsingContent(
+    onBack: () -> Unit,
+    modifier: Modifier = Modifier,
+) {
+    CollapsingTopBarScaffold(
+        modifier = modifier,
+        scrollMode = CollapsingTopBarScaffoldScrollMode.collapse(expandAlways = false),
+        topBar = {
+            SampleTopBarImage(
+                dog = CollapsingTopBarSampleDogDefaults.Advanced,
+            )
+
+            SampleTopAppBar(
+                title = "Body Resize",
+                onBack = onBack,
+                containerColor = Color.Transparent,
+            )
+        },
+        body = {
+            GradientContent(
+                modifier = Modifier.resizeWithCollapse(),
+            )
+        },
+    )
+}
+
+@Composable
+private fun GradientContent(
+    modifier: Modifier = Modifier,
+) {
+    val colorScheme = MaterialTheme.colorScheme
+    val gradient = remember(colorScheme) {
+        Brush.verticalGradient(
+            colors = listOf(
+                colorScheme.primary,
+                colorScheme.surface,
+                colorScheme.tertiaryContainer,
+            ),
+        )
+    }
+
+    Box(
+        modifier = modifier
+            .fillMaxSize()
+            .background(gradient)
+            .verticalScroll(rememberScrollState()),
+        contentAlignment = Alignment.Center,
+    ) {
+        Text("Center")
+    }
+}
+
+@Preview
+@Composable
+private fun Preview() {
+    ComposeCollapsingTopBarTheme {
+        BodyResizeSampleContent(
+            onBack = {},
+        )
+    }
+}