compose/ui/web: fix incorrect interpretation of ontouchend events
#2490
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eymar
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Oct 16, 2025
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Would you like to add a demo that you have created in the bug report? https://youtrack.jetbrains.com/issue/CMP-9030/wasm-canvas-incorrect-handling-of-multi-touch-input
It can be added here
https://github.com/JetBrains/compose-multiplatform-core/tree/jb-main/compose/mpp/demo
compose/ui/ui/src/webCommonW3C/kotlin/androidx/compose/ui/window/ComposeWindow.w3c.kt
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Thank you for your review. I have applied your suggestion.
I'd like to keep the changeset small if that's fine for you ! I don't think the demo is needed in this case |
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Thank you for your contribution! LGTM |
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In ComposeWindow.w3c.kt, the touch handler does not correctly handle the
touchendevent. Indeed the logic selecting betweentargetTouchesandchangedTouchesis flawed. Fortouchendboth should be taken into account, alsochangedTouchescontains the un-pressed pointer andtargetTouchescontains the rest of the remaining pressed pointers.The fix consists in removing the logic that compares the size of both
targetTouchesandchangedTouchesto pick the largest one.Instead, it checks the pointer event type to determine whether only
targetTouchesor bothtargetTouchesandchangedTouchesare needed while correctly accounting which touches are pressed depending on the source.Fixes: https://youtrack.jetbrains.com/issue/CMP-9030/wasm-canvas-incorrect-handling-of-multi-touch-input
Testing
I have added a failing test in
GestureTest.ktwhich is subsequently fixed by the implementation commit. The test consists in using three touches that are progressively pressed (1, 2, 3) and unpressed (3, 2, 1), logging and comparing the behavior of the pointerInput modifier with the expected events.Release Notes
Fixes - Web
ontouchendevents.