setBackgroundColor on QWebEnginePage dosen't seem to work consistently

setBackgroundColor seems to be broken for me when viewing multiple 'types of link'.

To reproduce on simplebrowser:

1. Apply attached patches to simplebrowser (to set background color to black), and print a little debug information.
2. Open a new blank tab with right click in tabbar -> new tab
3. Navigate to file:///dev/null , which displays a completely black page.
4. In the same tab, type 'qt.io' into the search bar, and hit enter.
5. Navigate to file:///dev/null in the same tab.

I expect this to display a black page, but instead, I see a white page.

Furthermore, once this state is reached, I see white flashes between loads of webpages (which this setting is supposed to prevent). 

Trying to workaround this issue by running setBackgroundColor repeatedly dosen't help, I must run it twice (to unset it, and set it back). Once doing that, it seems to work as expected, until it breaks again.

Please let me know if I can provide any additional information.