Changing the background of a QWebEnginePage via setBackground(..) to transparent changes the order in which the overlaying widgets are displayed. In the following example there is a LineEdit displayed over the QWebEngineView, this works as expected. Changing the background color to of the QWebEnigePage to QT::transparent renders the background transparent but also draws the QWebEngineView now over the LineEdit. In addition to that the area where the LineEdit would be still emits mouse events for the LineEdit.

Commenting/Uncommenting line 37 in ui_mainwindow.h changes between the two behaviors.