WebAssembly multi canvas "Failed to make context current"

balping

I'm trying to use multiple canvases / screens to render multiple QQuickView windows in wasm. I'm using Qt 6.3.2

// main.cpp
#include <QGuiApplication>
#include <QQuickView>

int main(int argc, char *argv[]){
	QGuiApplication app(argc, argv);

	const auto screens = app.screens();
	if(screens.size() < 2){
		qDebug() << "Expected at least 2 screens";
		return 1;
	}

	for(auto & screen : screens){
		qDebug() << screen;
	}

	QQuickView mainViewer;
	mainViewer.setScreen(screens[0]);
	mainViewer.setSource(QUrl(u"qrc:/wasm-multiwindow/main.qml"_qs));
	mainViewer.setResizeMode(QQuickView::SizeRootObjectToView);


	QQuickView secondViewer;
	secondViewer.setScreen(screens[1]);
	secondViewer.setSource(QUrl(u"qrc:/wasm-multiwindow/second.qml"_qs));
	secondViewer.setResizeMode(QQuickView::SizeRootObjectToView);

	mainViewer.show();
	secondViewer.show();


	return app.exec();
}

In the browser's console I get the following error:

QRhiGles2: Failed to make context current. Expect bad things to happen.
Failed to start frame

If I set the same screen for both QQuickView objects, there's no error, but only one of them is visible of course. I have no issue running this code on x64 Linux target with two physical displays.

So is this a bug

in the browser?
in Qt?
in my code?

If it's option 3, maybe I could try to add manual locking around the opengl context?

Also, I have no problem creating 2 widgets on 2 canvases:

// this works
#include <QApplication>
#include <QWidget>


int main(int argc, char *argv[]){
	QApplication app(argc, argv);

	const auto screens = app.screens();
	if(screens.size() < 2){
		qDebug() << "Expected at least 2 screens";
		return 1;
	}

	QWidget w1;
	w1.setScreen(screens[0]);
	w1.show();

	QWidget w2;
	w2.setScreen(screens[1]);
	w2.show();

	return app.exec();
}

For completeness here's rest of my code:

# wasm-multiwindow.pro
QT += quick

SOURCES += \
        main.cpp

resources.files = main.qml second.qml
resources.prefix = /$${TARGET}
RESOURCES += resources

# Additional import path used to resolve QML modules in Qt Creator's code model
QML_IMPORT_PATH =

# Additional import path used to resolve QML modules just for Qt Quick Designer
QML_DESIGNER_IMPORT_PATH =

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

// main.qml
import QtQuick

Rectangle {
    width: 640
    height: 480
    visible: true
    color: "#ff0000"

    Text {
        text: "hello"
    }
}

// second.qml
import QtQuick

Rectangle {
    width: 640
    height: 480
    visible: true
    color: "#00ff00"

    Text {
        text: "hi"
    }
}

This is the default autogenerated HTML, with an extra canvas inserted.

<!doctype html>
<html lang="en-us">
  <head>
    <meta charset="utf-8">
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">

    <!--Set visual viewport size for mobile devices to the device size,
        witch results in a scale of 1 and a 1:1 mapping between CSS pixels
        and Qt device independent pixels. -->
    <meta name="viewport" content="width=device-width, height=device-height, user-scalable=0"/>

    <title>wasm-multiwindow</title>
    <style>
      /* Make the html body cover the entire (visual) viewport with no scroll bars. */
      html, body { padding: 0; margin: 0; overflow:hidden; height: 100% }
      /* the canvas *must not* have any border or padding, or mouse coords will be wrong */
      canvas { border: 0px none; background-color: white; height:50%; width:100%;  }
      /* The contenteditable property is set to true for the canvas in order to support
         clipboard events. Hide the resulting focus frame and set the cursor back to
         the default cursor. */
      canvas { outline: 0px solid transparent; caret-color: transparent; cursor:default }
    </style>
  </head>
  <body onload="init()">
    <figure style="overflow:visible;" id="qtspinner">
      <center style="margin-top:1.5em; line-height:150%">
        <img src="qtlogo.svg" width="320" height="200" style="display:block"></img>
        <strong>Qt for WebAssembly: wasm-multiwindow</strong>
        <div id="qtstatus"></div>
        <noscript>JavaScript is disabled. Please enable JavaScript to use this application.</noscript>
      </center>
    </figure>
    <canvas id="qtcanvas" oncontextmenu="event.preventDefault()" contenteditable="true"></canvas>
    <canvas id="qtcanvas2" oncontextmenu="event.preventDefault()" contenteditable="true"></canvas>

    <script type='text/javascript'>
        function init() {
          var spinner = document.querySelector('#qtspinner');
          var canvas = document.querySelector('#qtcanvas');
          var canvas2 = document.querySelector('#qtcanvas2');
          var status = document.querySelector('#qtstatus')

          var qtLoader = QtLoader({
              canvasElements : [canvas, canvas2],
              showLoader: function(loaderStatus) {
                  spinner.style.display = 'block';
                  canvas.style.display = 'none';
                  status.innerHTML = loaderStatus + "...";
              },
              showError: function(errorText) {
                  status.innerHTML = errorText;
                  spinner.style.display = 'block';
                  canvas.style.display = 'none';
              },
              showExit: function() {
                  status.innerHTML = "Application exit";
                  if (qtLoader.exitCode !== undefined)
                      status.innerHTML += " with code " + qtLoader.exitCode;
                  if (qtLoader.exitText !== undefined)
                      status.innerHTML += " (" + qtLoader.exitText + ")";
                  spinner.style.display = 'block';
                  canvas.style.display = 'none';
              },
              showCanvas: function() {
                  spinner.style.display = 'none';
                  canvas.style.display = 'block';
              },
          });
          qtLoader.loadEmscriptenModule("wasm-multiwindow");
      }
    </script>
    <script type="text/javascript" src="qtloader.js"></script>
  </body>
</html>

And here's the full console output:

writeStackCookie: 432bd0 qtloader.js:396:25
checkStackCookie: 432bd0 2 qtloader.js:396:25
QML debugging is enabled. Only use this in a safe environment. qtloader.js:396:25
checkStackCookie: 432bd0 qtloader.js:396:25
InstallTrigger is deprecated and will be removed in the future. qtloader.js line 467 > eval:7386:6
QScreen(0x93dba8, name="qtcanvas") qtloader.js:396:25
QScreen(0x93e410, name="qtcanvas2") qtloader.js:396:25
Blocking on the main thread is very dangerous, see https://emscripten.org/docs/porting/pthreads.html#blocking-on-the-main-browser-thread qtloader.js:396:25
writeStackCookie: 980340 da626f5d-f10d-4e17-8d04-6b0ecadc878f:842:6
writeStackCookie: 1c82f20 6ecfa28e-e258-4b5e-9554-21200ff1c8f1:842:6
checkStackCookie: 432bd0 qtloader.js:396:25
QRhiGles2: Failed to make context current. Expect bad things to happen. qtloader.js:396:25
Failed to start frame qtloader.js:396:25
checkStackCookie: 432bd0 qtloader.js:396:25
main loop exiting.. qtloader.js:396:25
QRhiGles2: Failed to make context current. Expect bad things to happen. qtloader.js:396:25
Failed to start frame qtloader.js:396:25

balping

It is indeed a bug, see: https://bugreports.qt.io/browse/QTBUG-106707