On Mac the OS provides this feature (mostly) for free. We just added the standard menu entries and made sure it wasn’t messing up the custom title bars.
We have not implemented our own tabs for Linux or Windows for Merge yet, although that is on the roadmap. I don’t know that we would for Sublime Text since it already has the concept of tabs, and trying to re-architect the internals of Sublime Text to have multiple levels of tabs doesn’t necessarily seem like it is something we’d want to put time into.