QtWebEngine displays color banding on Wayland with QT_WAYLAND_DISABLE_WINDOWDECORATION=1

QtWebEngine-based browsers (this issue was originally discovered in Qutebrowser - related ticket 1, related ticket 2) seem to experience weird color banding when having the variable `QT_WAYLAND_DISABLE_WINDOWDECORATION` set to `1`. This doesn't seem to happen when window decorations are suppressed through the Wayland compositor, such as Sway.

This issue was deemed an upstream bug and as such I am reporting it here. Please note that this is my first time working with the Qt bugtracker or reporting bugs to Qt and as such do not hesitate to reach out for more information in comments.