I frequently combine and split the tabs in my windows for various reasons, and I would greatly prefer using a hotkey for this instead of a mouse click-drag.
I looked through the menus and online and found nothing, I analyzed the logged commands, which was just a regular drag event, and even tried a macro which I don’t think captured the action, since I could not save it (nothing happened, so I assume the recording was empty).
Detaching a tab to a new window is straightforward, but the reverse would probably have to assume that the last focused window (excluding the current) is the merge target.