AcceptNavigationRequest, loadStarted, loadProgress loadFinished sometimes aren't called

In some situations, QWebEnginePage::acceptNavigationRequest() is not called and the loadStarted(), loadProgress() and loadFinished() signals aren't emitted, neither from QWebEnginePage nor from QWebEngineView.

An example of when this happens is the following:

• go to https://github.com/KDE/konqueror
• click on the "Pull requests" link

The pull request page is correctly displayed, but the none of the debug messages are shown. Note that if you enter the URL of the pull request page directly in the URL bar of the browser, the signals are correctly emitted.

You can see this behavior using the simplebrowser example included with QtWebEngine, adding some calls to qDebug. I attach modified versions of the webpage.cpp, webpage.h and webview.cpp files included in the example which contains such calls.

This behavior makes it impossible for the program to detect when a page has actually finished loading, causing, for example, the bug reported at https://bugs.kde.org/show_bug.cgi?id=467850.