Comment by hombre_fatal

Anything from WhatsApp to the Settings app on macOS where I might want to, say, compare the wifi network settings for two access points.

But the point is that the developer has to build the app in a way that lets you do this.

Once again, there is no default with the tools I listed. You decide on how you want windowing to work and then build it.

At this point, show me examples of needing a mutex to "block" a multi-window default.