I don't know for sure if this is due to GTK weirdness but on certain applications the header and sidebars of the app are fine, but the main window pane is entirely transparent, with the cursor (and any text) leaving ghosts and failing to vanish. It makes the application entirely unusable and in one case I was able to fix this simply by defining the defaults to have a "white" background. (Rather than whatever was the default.)
I'm running a fairly vanilla, updated version of Lubuntu 18.04LTS. The programs with this issue are Evince (its postit notes), Bluefish, and RedNotebook...and some others, I'm quite sure.
Does anyone know why this is happening or what I can do to fix it? Most programs don't allow one to change the default background...so they're just useless. If I install KDE will that solve it?
(I confess I don't really even know what GTK is, really, or what it does, or why this would be linked. Just someone else said, "It's probably a GTK issue" so I want to try to fix it.)
Thank you in advance for any and all help!