Fix nested scrolling on mouse wheel

compose/foundation/foundation/src/commonMain/kotlin/androidx/compose/foundation/gestures/Scrollable.kt

@@ -587,7 +587,7 @@ internal interface ScrollConfig {
      * Enables animated transition of scroll on mouse wheel events.
      */
     val isSmoothScrollingEnabled: Boolean
-        get() = false
+        get() = true
 
     fun isPreciseWheelScroll(event: PointerEvent): Boolean = false
 

compose/foundation/foundation/src/jsWasmMain/kotlin/androidx/compose/foundation/gestures/JsScrollable.js.kt

@@ -30,10 +30,6 @@ import androidx.compose.ui.util.fastFold
 internal actual fun CompositionLocalConsumerModifierNode.platformScrollConfig(): ScrollConfig = JsConfig
 
 private object JsConfig : ScrollConfig {
-
-    override val isSmoothScrollingEnabled: Boolean
-        get() = true
-
     override fun Density.calculateMouseWheelScroll(event: PointerEvent, bounds: IntSize): Offset {
         // Note: The returned offset value here is not strictly accurate.
         // However, it serves two primary purposes:

compose/foundation/foundation/src/skikoTest/kotlin/androidx/compose/foundation/copyPasteAndroidTests/ScrollableTest.kt

@@ -768,17 +768,19 @@ class ScrollableTest {
 
     @OptIn(ExperimentalTestApi::class)
     @Test
-    fun scrollable_nestedScroll_disabledForMouseWheel() = runSkikoComposeUiTest {
+    fun scrollable_nestedScroll_childPartialConsumptionForMouseWheel() = runSkikoComposeUiTest {
         var innerDrag = 0f
         var outerDrag = 0f
         val outerState = ScrollableState(
             consumeScrollDelta = {
+                // Since the child has already consumed half, the parent will consume the rest.
                 outerDrag += it
                 it
             }
         )
         val innerState = ScrollableState(
             consumeScrollDelta = {
+                // Child consumes half, leaving the rest for the parent to consume.
                 innerDrag += it / 2
                 it / 2
             }
@@ -812,7 +814,10 @@ class ScrollableTest {
         }
         runOnIdle {
             assertThat(innerDrag).isGreaterThan(0f)
-            assertThat(outerDrag).isEqualTo(0f)
+            assertThat(outerDrag).isGreaterThan(0f)
+            // Since child (inner) consumes half of the scroll, the parent (outer) consumes the
+            // remainder (which is half as well), so they will be equal.
+            assertThat(innerDrag).isEqualTo(outerDrag)
         }
     }
 

compose/mpp/demo/src/commonMain/kotlin/androidx/compose/mpp/demo/components/Components.kt

@@ -56,5 +56,6 @@ val Components = Screen.Selection(
     LazyLayouts,
     MaterialComponents,
     Material3Components,
+    Screen.Example("NestedScroll") { NestedScrollExample() },
     Screen.Example("Selection") { SelectionExample() }
 )

compose/mpp/demo/src/commonMain/kotlin/androidx/compose/mpp/demo/components/NestedScroll.kt

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * 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 androidx.compose.mpp.demo.components
+
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.PaddingValues
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.material.Text
+import androidx.compose.material.TopAppBar
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.input.nestedscroll.NestedScrollConnection
+import androidx.compose.ui.input.nestedscroll.NestedScrollSource
+import androidx.compose.ui.input.nestedscroll.nestedScroll
+import androidx.compose.ui.platform.LocalDensity
+import androidx.compose.ui.unit.IntOffset
+import androidx.compose.ui.unit.dp
+import kotlin.math.roundToInt
+
+// Copy from compose/ui/ui/samples/src/main/java/androidx/compose/ui/samples/NestedScrollSamples.kt
+@Composable
+fun NestedScrollExample() {
+    // here we use LazyColumn that has build-in nested scroll, but we want to act like a
+    // parent for this LazyColumn and participate in its nested scroll.
+    // Let's make a collapsing toolbar for LazyColumn
+    val toolbarHeight = 48.dp
+    val toolbarHeightPx = with(LocalDensity.current) { toolbarHeight.roundToPx().toFloat() }
+    // our offset to collapse toolbar
+    val toolbarOffsetHeightPx = remember { mutableStateOf(0f) }
+    // now, let's create connection to the nested scroll system and listen to the scroll
+    // happening inside child LazyColumn
+    val nestedScrollConnection = remember {
+        object : NestedScrollConnection {
+            override fun onPreScroll(available: Offset, source: NestedScrollSource): Offset {
+                // try to consume before LazyColumn to collapse toolbar if needed, hence pre-scroll
+                val delta = available.y
+                val newOffset = toolbarOffsetHeightPx.value + delta
+                toolbarOffsetHeightPx.value = newOffset.coerceIn(-toolbarHeightPx, 0f)
+                // here's the catch: let's pretend we consumed 0 in any case, since we want
+                // LazyColumn to scroll anyway for good UX
+                // We're basically watching scroll without taking it
+                return Offset.Zero
+            }
+        }
+    }
+    Box(
+        Modifier
+            .fillMaxSize()
+            // attach as a parent to the nested scroll system
+            .nestedScroll(nestedScrollConnection)
+    ) {
+        // our list with build in nested scroll support that will notify us about its scroll
+        LazyColumn(contentPadding = PaddingValues(top = toolbarHeight)) {
+            items(100) { index ->
+                Text("I'm item $index", modifier = Modifier.fillMaxWidth().padding(16.dp))
+            }
+        }
+        TopAppBar(
+            modifier = Modifier
+                .height(toolbarHeight)
+                .offset { IntOffset(x = 0, y = toolbarOffsetHeightPx.value.roundToInt()) },
+            title = { Text("toolbar offset is ${toolbarOffsetHeightPx.value}") }
+        )
+    }
+}

compose/ui/ui/src/desktopTest/kotlin/androidx/compose/ui/input/mouse/MouseMoveTest.kt

@@ -371,7 +371,7 @@ class MouseMoveTest {
         val collector2 = EventCollector()
 
         setContent {
-            LazyColumn(Modifier.size(10.dp)) {
+            LazyColumn(Modifier.size(12.dp)) {
                 items(2) {
                     Box(
                         modifier = Modifier
@@ -382,16 +382,16 @@ class MouseMoveTest {
             }
         }
 
-        scene.sendPointerEvent(PointerEventType.Enter, Offset(0f, 0f))
+        scene.sendPointerEvent(PointerEventType.Enter, Offset(5f, 5f))
         collector1.assertCounts(enter = 1, exit = 0)
         collector2.assertCounts(enter = 0, exit = 0)
 
-        scene.sendPointerEvent(PointerEventType.Scroll, Offset(0f, 0f), scrollDelta = Offset(0f, 10000f))
+        scene.sendPointerEvent(PointerEventType.Scroll, Offset(5f, 5f), scrollDelta = Offset(0f, 10000f))
         waitForIdle()
         collector1.assertCounts(enter = 1, exit = 1)
         collector2.assertCounts(enter = 1, exit = 0)
 
-        scene.sendPointerEvent(PointerEventType.Scroll, Offset(0f, 0f), scrollDelta = Offset(0f, -10000f))
+        scene.sendPointerEvent(PointerEventType.Scroll, Offset(5f, 5f), scrollDelta = Offset(0f, -10000f))
         waitForIdle()
         collector1.assertCounts(enter = 2, exit = 1)
         collector2.assertCounts(enter = 1, exit = 1)