[5.12] QWebEnginePage::createWindow hides new tab/window when closed

When a new tab or window is opened by the QWebEnginePage::createWindow or QWebEngineView::createWindow methods, the new window or tab will display properly at first.

However, when the page that called the createWindow method is destroyed, all of the pages created by createWindow will become invisible and cannot be interacted with.

These pages still seem to exist (signals for the page being loaded or the title being changed are still emitted), but the QWebEngineView will not render the pages.

To confirm this, open the attached test.html file in simplebrowser,  click on the link which will open a new tab, then switch to the new tab and close the first tab. You will see the contents of the page in the second tab disappear.