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.

Excuse me for my glitchy network...

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

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 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"…