Earlier quoted context omitted.
Which desktop? It's not true for AppKit, SwiftUI, GTK, Qt, WinUI, or Electron, and I'm not sure what's left.
What? 99% of my programs I can open more than one window by just executing the program as normal multiple times, the only ones that enforce a kind of singleton execution are things that also acts as a kind of server, one example is Everything search, but it has a "Open new Search window" for multiple windows. And that is the default, you have to explicit code in a lock/global mutex thing to block multiple instances.…
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.