Comment 3 for bug 1599453

Hi Braden, a couple more possibilities:

1. Your GTK 3.0 will more likely be GTK 3.18 in 16.04, unless you have been installing PPA's or compiling your own GTK. Can you double check the version. I'm running 14.04 (previous LTS) with GTK 3.10, so perhaps default window/widget background behaviour has changed from not doing anything (i.e. empty and transparent) to filled with the theme's default background colour.

Gnome/GTK devs are notorious for making significant changes between 3.x releases. It is perfectly possible that someone decided "this is wrong" and just changed it. Then us downstream people end up chasing our tails trying to figure out why different people have different behaviour.

2. Another possibility is your theme. Perhaps it is making the background of the window solid gray instead of clear. Which theme are you using? (I'm using default Ubuntu Ambiance) And what happens if you try others? (This might take some effort to get them installed and set.)

I may have to spin up a 16.04 VM if none of this sheds light on things.