Desktop multi-window support

[yjbanov]

Document Link

https://flutter.dev/go/desktop-multi-window-support

What problem are you solving?

In order to implement many desirable features on desktop platforms, it is necessary to be able to open and manage multiple window contexts. This document describes the API and mechanisms for that.

[gspencergoog]

This design doc is now out of date: the described changes were made a long time ago, and have been superseded by updated designs and implementations.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.