Thanks for testing, @Developer Tools Engineer. No, I didn't see this issue as long as the Xcode window isn't fullscreen, i. e. no window popped up. All I noticed is a hidden window described in the screenshot above this post: I also noticed that the mechanism behind the problem also works when the Xcode window is not full-screen. Those windows are just "hidden"…

Thanks for responding. The defaults I mentioned is about any way to access macOS preferences subsystem (.plist, including NSUserDefaults). To enable Metal HUD for apps I need to monitor, I just set MetalForceHudEnabled to YES for those apps, once. However, if I want to customize what to show in Metal HUD 4, I have to use open with MTL_HUD_ELEMENTS because I haven't found its plist key yet.

Thanks for helping. I missed something and now worked out solution for my needs.

Excuse me for my glitchy network...

I'm encountering it too. I've not ever noticed it before and don't feel much performance loss. I deleted "DocumentationCache" and "DocumentationIndex" at least.