(Qt for WebAssembly) MultiPointTouchArea loses all touchpoints after just one is released - qt

Question

If I run the following code in the browser using Qt for webassembly I can shift points around, but if one of the two is released, shifting no longer works and this console messages appears:
qt.pointer.dispatch: skipping QEventPoint(id=1 ts=0 pos=0,0 scn=869,370 gbl=869,370 Updated ellipse=(8x8 ∡ 0) vel=0,0 press=-869,-370 last=-869,-370 Δ 869,370) : no target window
import QtQuick
Window {
visible: true
Rectangle {
anchors.fill:parent
MultiPointTouchArea {
anchors.fill: parent
touchPoints: [
TouchPoint {
id: point1
onPressedChanged: console.log('point1', pressed);
},
TouchPoint {
id: point2
onPressedChanged: console.log('point2', pressed);
}
]
}
Rectangle {
width: 200; height: width;
color: "red"
x: point1.x
y: point1.y
}
Rectangle {
width: 200; height: width;
color: "green"
x: point2.x
y: point2.y
}
}
}
I tested it with versions Qt5.15.2, Qt6.4.2, Qt6.5
and different browsers, but the same error occurs all the time.
Link "test it yourself" Online live qml wasm
I debugged a little and it seems the error is in file:
C:\Qt\6.5.0\Src\qtbase\src\gui\kernel\qguiapplication.cpp
Line 2924:
QPointer<QWindow> window = e->window; // the platform hopefully tells us which window received the event
Line 2961:
// If we somehow still don't have a window, we can't deliver this touchpoint. (should never happen)
if (Q_UNLIKELY(!window)) {
qCWarning(lcPtrDispatch) << "skipping" << &tempPt << ": no target window";
continue;
}
here the window is lost.
How can i debug it further?
I also tested it with Qt under Windows, Linux, Android using diffrent versions. There everything works as expected (only using WebAssembly in the browser it doesn't)

Answer 1

If you check the signals of MultiPointTouchArea you see that you get passed a touchPoints array as an argument. This array is valid during the signal handler, and, you need to react accordingly. The following code runs in WebAssembly and you can test it out on a multitouch device:
import QtQuick
import QtQuick.Controls
Page {
background: Rectangle { color: "#848895" }
property list<string> status
MultiPointTouchArea {
anchors.fill: parent
onCanceled: msg(`onCanceled: ${touchPoints.length}`);
onPressed: msg(`onPressed: ${touchPoints.length}`);
onGestureStarted: msg(`onGestureStarted: ${touchPoints.length}`);
onReleased: msg(`onReleased: ${touchPoints.length}`);
onTouchUpdated: msg(`onTouchUpdated: ${touchPoints.length}`);
onUpdated: msg(`onUpdated: ${touchPoints.length}`);
Frame {
anchors.centerIn: parent
background: Rectangle { }
Text {
text: status.join('\n');
}
}
}
function msg(str) {
status.push(str);
if (status.length > 20) status.shift();
}
}
You can Run it WebAssembly now