JSON serialization of the WindowProxy object isn't very useful if it's a nested browser context

If the WindowProxy is a nested browsing context, you get a window handle, except you can't actually use it for anything (e.g., you can't use it to switch the current browsing context). Having an opaque string that's only useful for comparing it doesn't seem like it is very useful?

(Technically, you could, by going to its top-level browsing context and then switching to every frame until you find one with the same window handle.)

As for a use-case: I have a message event and I want to switch the current browsing context to e.source to react to that message.

Start with the issue's discussion of WindowProxy serialization, nested browsing contexts, window handles, and switching contexts. Review the WebDriver browsing-context and window-handle behavior, then determine a concrete protocol change that would let a message event's source identify a usable context; done means the behavior and interoperability expectations are specified.