If ran without compositing on, CSD windows show a large black border around the entire window.
Even ones without a header back will do that. For example, run "GTK_CSD=1 xfce4-settings-manager".
If "Enable display compositing" is disabled, those windows show an ugly black border all around the window.
So please explicitly enable compositing for such windows and only disable it for non CSD windows.
I mean to type "Even ones without a header bar will do that".
Compositing is a global thing, it's not per window.
Ok, thank you for the reply.
At some point though, it will need to be permanently enabled as 4.16 will start experimenting with client side decorations according to https://simon.shimmerproject.org/2019/10/19/xfce-4-15-development-phase-starting/
GTK supports CSD without compositing, so not required.
In this screenshot https://i.imgur.com/t0lD6AW.png:
1) the ssd windows look ok without compositing.
2) The windows with headerbars look bad without compositing (GTK_CSD is not set).
3) Non header bar windows look bad with GTK_CSD=1.
Basically, the issue is happening for windows in point 2.
(In reply to Hussam Al-Tayeb from comment #0)
> If ran without compositing on, CSD windows show a large black border around
> the entire window.
> If "Enable display compositing" is disabled, those windows show an ugly
> black border all around the window.
I see where the confusion comes from here...
As I said in comment 4, GTK supports CSD without a compositor  but it does not monitor the property continuously (as very window managers/compostior are able to enable/disable compositing on the fly), so you need to restart the GTK applications if you enable/disable compositing.
Thy this instead:
1. Disable compositing
2. Start gtk3-demo after
3. Check CSD windows
Ok, got it. Thank you. I wasn't restarting GTK applications after toggling the composting option.