Redispatch unconsumed mouse wheel events #2425

m-sasha · 2025-09-30T08:55:34Z

Redispatch mouse wheel events which the Compose scene did not consume.

Difference with Swing behavior

This fix introduces a slight difference with how Swing handles nested mouse-wheel scrolling. In Swing, there is essentially no nested scrolling. In a scenario with a JScrollPane inside another JScrollPane, only the inner one can be scrolled when the mouse is over it. With this PR, a ComposePanel with a scrollable element within a JScrollPane will scroll until it reaches the start/end and then the outer JScrollPane will scroll.
This behavior seems to be an improvement, so we thought it was ok to have this difference.

Danger, Will Robinson

Note that this includes a potentially dangerous hack of whose consequences I'm not yet sure. If it proves too problematic, we will need to take a different approach (documented in the source).
The hack is that in order to allow AWT to find the correct target for the redispatched event, we temporarily unregister ComposeSceneMediator's mouse listeners.

Feature flag

I've put the behavior behind ComposeFeatureFlags.redispatchUnconsumedMouseWheelEvents. The flag will be:

false by default in 1.9.1
true by default in 1.10
Removed in 1.11, if no major problems are discovered.
In this PR, it's true, as it merges into jb-main. In the cherry-pick to 1.9.1, we should change it to false. When it's approved I will add a ticket to remove it for 1.11

Other mouse events

Note that this PR only deals with mouse wheel events, but at least in the case of a lightweight skia layer (i.e. SkiaSwingLayer), we should probably do the same for other mouse events.

Fixes https://youtrack.jetbrains.com/issue/CMP-4601

Testing

Added unit tests, and also tested manually with

import androidx.compose.foundation.*
import androidx.compose.foundation.layout.*
import androidx.compose.material.*
import androidx.compose.runtime.Composable
import androidx.compose.ui.*
import androidx.compose.ui.awt.*
import androidx.compose.ui.graphics.*
import androidx.compose.ui.unit.*
import java.awt.*
import javax.swing.*

fun main() = SwingUtilities.invokeLater {
    val frame = JFrame("CfD repro")
    frame.defaultCloseOperation = JFrame.EXIT_ON_CLOSE
    frame.minimumSize = Dimension(500, 400)
    System.setProperty("compose.swing.render.on.graphics", "true")

    val mainPanel =
        JPanel(null).apply {
            val container =
                JPanel(null).apply {
                    layout = BoxLayout(this, BoxLayout.Y_AXIS)
                    addChildren()
                }
            val scrollPane = JScrollPane(container)
            scrollPane.setBounds(0, 0, 500, 500)
            add(scrollPane)
        }

    frame.contentPane.add(mainPanel, BorderLayout.CENTER)
    frame.isVisible = true
}

private fun JPanel.addChildren() {
    repeat(10) {
        repeat(5) {
            add(JLabel("<html>" + "Very long text here ".repeat(10) + "</html>"))
            add(Box.createVerticalStrut(10))
        }

        add(
            ComposePanel().apply {
                setContent {
                    ComposeBox()
                }
            }
        )
        add(Box.createVerticalStrut(10))
    }
}

@Composable
fun ComposeBox() {
    Box(
        modifier = Modifier
            .height(100.dp)
            .background(color = Color(180, 180, 180))
            .padding(10.dp)
    ) {
        val stateVertical = rememberScrollState(0)

        Box(
            modifier = Modifier
                .verticalScroll(stateVertical)
                .padding(end = 12.dp, bottom = 12.dp)
        ) {
            Column {
                for (item in 0..10) {
                    TextBox("Item #$item")
                }
            }
        }
    }
}

@Composable
fun TextBox(text: String = "Item") {
    Box(
        modifier = Modifier.height(32.dp)
            .width(400.dp)
            .background(color = Color(0, 0, 0, 20))
            .padding(start = 10.dp),
        contentAlignment = Alignment.CenterStart
    ) {
        Text(text = text)
    }
}

This should be tested by QA

Release Notes

Fixes - Desktop

ComposePanel now re-dispatches unconsumed mouse wheel events, allowing scrollable components beneath to be scrolled. To disable this, set the system property "compose.swing.redispatchMouseWheelEvents" to "false".

hoc081098 · 2025-09-30T16:16:03Z

compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/scene/ComposeSceneMediator.desktop.kt

+    /**
+     * Returns the first heavyweight ancestor of the given component.
+     */
+    private fun Component.heavyWeightAncestorOrNull() : Component? {


Nitpick: it could be a top-level function

igordmn · 2025-10-01T19:31:39Z

Adding a second reviewer, as it is a new API (even as a flag) (UPDATED: potential API via System.setProperty)

MatkovIvan · 2025-10-01T19:37:59Z

compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/ComposeFeatureFlags.desktop.kt

+     * This flag will be removed in the future, and the default behavior will correspond to a value
+     * of `true`.
+     */
+    val propagateUnconsumedMouseWheelEvents = FeatureFlag { true }


How it's supposed to be changed outside? I guess it should be System.getProperty, right?

Yeah, I forgot FeatureFlag was not mutable.

Would you prefer I made FeatureFlag mutable (and public) or just add a system property?

Added a system property.

This class is enttirely internal. No, we don't need it in public

compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/ComposeFeatureFlags.desktop.kt

MatkovIvan

No blockers from me. Please wait @igordmn approval

MatkovIvan · 2025-10-02T10:56:53Z

compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/scene/ComposeSceneMediator.desktop.kt

-        component.addFocusListener(focusListener)
-        component.addKeyListener(keyListener)
-        component.subscribeToMouseEvents(mouseListener)
+    private fun subscribeToInputEvents() {


just curiosity, what was wrong with previous shape here?

You mean that it used to take the component to register with as a receiver?
It wasn't needed and was error prone, as the component has to be the same one in all cases.

igordmn · 2025-10-02T11:24:46Z

compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/ComposeFeatureFlags.desktop.kt

+     * of `true`.
+     */
+    val propagateUnconsumedMouseWheelEvents = FeatureFlag {
+        System.getProperty("compose.swing.propagateMouseWheelEvents", "true").toBoolean()


Please add this property to the release notes.

Also if we switch it to false in 1.9.1, we have to change the release notes in the cherry-pick.

This is a cherry-pick (with some adaptations) of #2425 Redispatch mouse wheel events which the Compose scene did not consume. ### Difference with Swing behavior This fix introduces a slight difference with how Swing handles nested mouse-wheel scrolling. In Swing, there is essentially no nested scrolling. In a scenario with a `JScrollPane` inside another `JScrollPane`, only the inner one can be scrolled when the mouse is over it. With this PR, a `ComposePanel` with a scrollable element within a `JScrollPane` will scroll until it reaches the start/end and then the outer `JScrollPane` will scroll. This behavior seems to be an improvement, so we thought it was ok to have this difference. ### Danger, Will Robinson Note that this includes a potentially dangerous hack of whose consequences I'm not yet sure. If it proves too problematic, we will need to take a different approach (documented in the source). The hack is that in order to allow AWT to find the correct target for the redispatched event, we temporarily unregister `ComposeSceneMediator`'s mouse listeners. ### Feature flag I've put the behavior behind `ComposeFeatureFlags.redispatchUnconsumedMouseWheelEvents`. The flag will be: - `false` by default in 1.9.1 - `true` by default in 1.10 - Removed in 1.11, if no major problems are discovered. In this PR, it's `true`, as it merges into `jb-main`. In the cherry-pick to 1.9.1, we should change it to `false`. When it's approved I will add a ticket to remove it for 1.11 ### Other mouse events Note that this PR only deals with mouse wheel events, but at least in the case of a lightweight skia layer (i.e. `SkiaSwingLayer`), we should probably do the same for other mouse events. Fixes https://youtrack.jetbrains.com/issue/CMP-4601 ## Testing Added unit tests, and also tested manually with ``` import androidx.compose.foundation.* import androidx.compose.foundation.layout.* import androidx.compose.material.* import androidx.compose.runtime.Composable import androidx.compose.ui.* import androidx.compose.ui.awt.* import androidx.compose.ui.graphics.* import androidx.compose.ui.unit.* import java.awt.* import javax.swing.* fun main() = SwingUtilities.invokeLater { val frame = JFrame("CfD repro") frame.defaultCloseOperation = JFrame.EXIT_ON_CLOSE frame.minimumSize = Dimension(500, 400) System.setProperty("compose.swing.render.on.graphics", "true") val mainPanel = JPanel(null).apply { val container = JPanel(null).apply { layout = BoxLayout(this, BoxLayout.Y_AXIS) addChildren() } val scrollPane = JScrollPane(container) scrollPane.setBounds(0, 0, 500, 500) add(scrollPane) } frame.contentPane.add(mainPanel, BorderLayout.CENTER) frame.isVisible = true } private fun JPanel.addChildren() { repeat(10) { repeat(5) { add(JLabel("<html>" + "Very long text here ".repeat(10) + "</html>")) add(Box.createVerticalStrut(10)) } add( ComposePanel().apply { setContent { ComposeBox() } } ) add(Box.createVerticalStrut(10)) } } @composable fun ComposeBox() { Box( modifier = Modifier .height(100.dp) .background(color = Color(180, 180, 180)) .padding(10.dp) ) { val stateVertical = rememberScrollState(0) Box( modifier = Modifier .verticalScroll(stateVertical) .padding(end = 12.dp, bottom = 12.dp) ) { Column { for (item in 0..10) { TextBox("Item #$item") } } } } } @composable fun TextBox(text: String = "Item") { Box( modifier = Modifier.height(32.dp) .width(400.dp) .background(color = Color(0, 0, 0, 20)) .padding(start = 10.dp), contentAlignment = Alignment.CenterStart ) { Text(text = text) } } ``` This should be tested by QA ## Release Notes ### Fixes - Desktop - ComposePanel can now re-dispatch unconsumed mouse wheel events, allowing scrollable components beneath to be scrolled. To enable this behavior, set the system property `"compose.swing.redispatchMouseWheelEvents"` to `"true"`.

m-sasha force-pushed the m-sasha/fix-mousewheel-event-propagation branch 6 times, most recently from 9a3021d to 06376d4 Compare September 30, 2025 14:56

m-sasha requested a review from igordmn September 30, 2025 15:13

hoc081098 reviewed Sep 30, 2025

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Redispatch unconsumed mouse wheel events

9b636d9

m-sasha force-pushed the m-sasha/fix-mousewheel-event-propagation branch from ea05b5a to ee607e3 Compare October 1, 2025 12:05

igordmn requested a review from MatkovIvan October 1, 2025 19:31

MatkovIvan reviewed Oct 1, 2025

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igordmn requested a review from MatkovIvan October 1, 2025 19:38

m-sasha force-pushed the m-sasha/fix-mousewheel-event-propagation branch from ee607e3 to dc1dac0 Compare October 2, 2025 10:21

MatkovIvan reviewed Oct 2, 2025

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compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/ComposeFeatureFlags.desktop.kt Outdated Show resolved Hide resolved

m-sasha force-pushed the m-sasha/fix-mousewheel-event-propagation branch from dc1dac0 to 6219dbb Compare October 2, 2025 10:42

MatkovIvan approved these changes Oct 2, 2025

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igordmn approved these changes Oct 2, 2025 •

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Add a flag to control the redispatching behavior

46f7c8f

m-sasha force-pushed the m-sasha/fix-mousewheel-event-propagation branch from 6219dbb to 46f7c8f Compare October 2, 2025 11:34

m-sasha merged commit 773fa7b into jb-main Oct 2, 2025
11 checks passed

m-sasha deleted the m-sasha/fix-mousewheel-event-propagation branch October 2, 2025 11:54

m-sasha added a commit that referenced this pull request Oct 2, 2025

Redispatch unconsumed mouse wheel events (#2425)

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m-sasha mentioned this pull request Oct 2, 2025

Redispatch unconsumed mouse wheel events (#2425) #2438

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m-sasha added a commit that referenced this pull request Oct 2, 2025

Redispatch unconsumed mouse wheel events (#2425)

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m-sasha commented Sep 30, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Difference with Swing behavior

Danger, Will Robinson

Feature flag

Other mouse events

Testing

Release Notes

Fixes - Desktop

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