Dropdown list options are detached from dropdown list control

The options you get to select from when you click on a dropdown list control are detached from the control, appearing further down the page. This doesn't happen on all dropdown lists.

Some dropdown lists exhibit this behaviour consistently. such as at http://www.betfred.com/sport. Scroll down the page to the price format dropdown list that displays 'Fractions' by default. Click on the dropdown list and the available options appear detached further down the page. See attached screenshot Screenshot_Betfred_falkon.png. This website may only be accessible from the UK or you could use a vpn. Other sites I have come across so far that contain dropdown lists that consistently exhibit this behaviour require an account login.

Some dropdown lists appear to consistently work correctly. Examples of this are Severity, Platform and OS on the KDE Bug Tracker.

The Maintainer and Flagged dropdown lists at https://www.archlinux.org/packages/ exhibit this behaviour intermittently. After Falkon is restarted, the dropdown lists behave correctly.

This was originally reported downstream against the Falkon web browser in the KDE bug tracker https://bugs.kde.org/show_bug.cgi?id=393556 where it was suggested the issue be reported upstream.

The dropdown lists work correctly when Falkon is built for qt5-webengine 5.10.1.