InputMethod not working on QQuickWidget/QWebEngineView embedded in QGraphicsProxyWidget

When placing a QWebEngineView in a QGraphicsProxyItem on a QGraphics Scene, InputMethods don't work in input fields of the displayed web page.

My analysis retrieved that the KeyPressEvents for dead keys are still sent to the widget instead of the InputMethodEvent of the composed character.

Strangely: After moving the item while an input field on the web page has the focus, input methods will work. So I assume that some synchronization through the QGraphicsProxyItem is missing, which is however triggered by moving the item.

I have attached a minimal proof-of-failure, showing the main page of the French Wikipedia. Click on the search button and try to input some text requiring a dead key. The dead key is ignored. Then move the item by pressing the "Move" button. Click again in the search input field and combined keys will work.