Will try to repro tomorrow using a Windows 10 VM.

That's just the regular Windows window border appearing.
On my test machine it look like this:

The color of the border is different because the Windows accent color is different from yours.

If you don't want that window border you can set WindowStyle=None, but you will loose the minimize/restore animation by doing so.
If you want the border to go all around your window you can also use the GlowWindowBehavior.
Or you can inherit your window from WindowChromeWindow and copy the style/template from

I don't get the artifact when just using Microsoft's WindowChrome, so a bit surprised I'm getting it with ControlzEx. Anyway, thank you for looking into it, at least I'm not missing something.

I now know why this happens with ControlzEx but not with WindowChrome.
ControlzEx does not extend the glass frame whereas WindowChrome does.
As i am currently adding support for Backdrop (Mica etc.) to ControlzEx and doing so requires extending the glass frame you will be able to get rid of those borders either in version 7 or version 8 of ControlzEx.

That's great to hear! I will keep an eye out and test the upcoming versions.