Let's say we have:

QWebEngineView *view;
QWebEnginePage *page1;
QWebEnginePage *page2;

And the view alternates between showing page1 and page2.

Each call like

// view is showing pageX
view->setPage(pageY);
// view is now showing pageY

will move the internal focus of pageX if the view had the focus when setPage() is called.

I have attached to this ticket a minimal app showing the issue and a video recording of this app.

My understanding of the causes is that when changing the page of a QWebEngineView, its internal QQuickWidget is hidden and deleted.

When it is hidden, QWidgetPrivate::hide_helper() is called.
Because the QQuickWidget being hidden has focus, QWidgetPrivate::hide_helper() will then call QQuickWidget::focusNextPrevChild().
Then QQuickWidget::focusNextPrevChild() will send a tab down and a tab up event to its internal QQuickView that will end up forwarded to the QWebEnginePage.

I was able to see the tab events in GammaRay and in Chromium, using JS to listen to key events in the DOM.