QEvents triggered twice in my QWebSocket

[kili236]

Hello everyone,

My application is using React for the frontend and C++ for the backend. The communication between both is implemented using QWebEngine and the QWebChannel.

I recently updated Qt from version 5.10.1 (msvc_2015) to 5.15.2 (msvc_2019) and I encountered a weird behavior after the update:
In the UI, I have a button used to trigger a BE signal opening a QDialog. If I start the application and click on this button, the QWebSocket will emit twice the signal textMessageReceived. But then, if I click again on the same button, it will fire only once. This behavior is 100% reproducible (meaning that if I restart the application and perform the same steps, it will happen again), but is also random because I couldn't link the behavior with a given component of my UI. This happens several times during the run of my application, for different signals.

I started investigating the stack in debug mode and noticed that the QEventDispatcherWin32 is posting events in the bool QEventDispatcherWin32::processEvents(QEventLoop::ProcessEventsFlags flags) function as well as in LRESULT QT_WIN_CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp), and that the events handled in void QCoreApplication::sendPostedEvents(QObject *receiver, int event_type) result both to the same message sent to my QWebSocket. I also analyzed the network to be sure that the signal is not triggered twice by the FE.

If you need it, I can add the call stacks.

I tried to reproduce a minimal example but, unfortunately, couldn't reproduce it.

Does anyone recognize the symptoms or has any idea of what is the issue?

Thank you very much