For a couple of weeks now I have seen this white border line and wondered if this is a glitch or intentional. I hope it was intentional as its a nice touch when running black on black. Using a modified blackMATE theme.
It’s the windows border you are using and i am pretty sure it is intentional. I forget the name of it, but I have noticed this “highlighting” effect on window borders when it is used. It’s not part of black mate, So it must be an after-the fact modification you have used. Indeed, I don’t even think it comes as part of the Ubuntu Mate installation set. Have you downloaded additional themes? The window border looks very much like part of a theme I have seen used on XFCE and LXDE.
One way to test this is to go back to the standard black mate and then rebuild it according to how it currently looks. At some point in the rebuilding process, you will identify which element is causing the white line effect so you will know how to reimplement it in the future,
Thanks steve
On 12o4 att, but yes I recall changing the border. Will try that later.