Due to prioritization of other work-items this has been moved to the backlog. We will update when it is being actively worked on again.
Multi-window chat is in testing. I hope to have great news regarding rollout to the public rings soon.
We are working to ensure that multi-window and other future multi-window experiences are of the highest quality when they release.
Once support for Windows 10 native notifications is released, users who choose to use Windows notifications will be able to utilize Windows OS specific settings for the length of time to display notifications. Mac does not have an OS specific setting, so will not have the support.
This feature item is still in the works. We will keep you posted as we have new info from the feature team.