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!

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    I don't know your issue but will provide a little background. LXDE used by Lubuntu 18.04 LTS and before uses GTK+2 (version 2 of the gimp+gnome toolkit), GNOME moved to v3 years ago (ie. GTK+3) but LXDE never did so it's using an older toolkit (the tk). Lubuntu switched from LXDE (pretty much abandoned) to LXQt (using the Q toolkit also used by KDE) at the 18.10 release (it was available prior as preview only; Lubuntu Next) Choosing themes that are compatible with all three (gtk+2, gtk+3 & Qt) is probably your easiest fix; Evince is GTK+3 for example so uses GTK+3 theming. – guiverc Nov 20 at 22:00
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    I may return later (when using LXDE and not my current LXQt) and provide more specific/useful help, but in the past I've just played with themes until I'm happy with the apps I want to use. – guiverc Nov 20 at 22:08
  • Thanks so much, @guiverc for the info, I really appreciate it. When you say "play around with themes" what exactly does that entail. Are we talking Ubuntu versus Lubuntu? Or is there something I can do within Lubuntu, some setting or app, that will help. I'm not married to the existing "theme" but it is generally fine. If I can change it, are there risks? Can I switch back easily if I don't like one? Thank you for any additional help you can offer. – user1149499 Nov 21 at 3:56
  • By "playing around" I meant making changes in Preferences->Customize.Look.and.Feel, or Preferences->OpenBox.Configuration.Manager (maybe even Preferences->Desktop.Preferences but less likely). ie. just change settings and see if it improves things (you'll see many effects straight away or if you hit "Apply"), but if you find your best choices, wait until you logout/login before you accept it's done. You just change it back to what it was before if you don't like it. You'll note though you can't change the evince buttons, as evince obey's [only] GNOME's settings (as it's a GNOME app) – guiverc Nov 21 at 6:29

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