Can't remember, it was a long time since I used it.
It was when OneDrive announced:
"Folder uploads via OneDrive.com
While today’s announcements mostly feature improvements to the desktop experience, there’s one more popular feature request we received for the OneDrive website. We’re pleased to announce that today we’re releasing the ability for you to drag folders directly into OneDrive.com from browsers where folder dragging/dropping is supported (specifically Google Chrome)." https://blog.onedrive.com/onedrive-now-supports-10-gb-files/
I am pretty sure that I could (and did) upload folders to OneDrive using Maxthon (Ultra Mode) when I first saw the OneDrive announcement.
Now I can't drag and drop anything in Ultra mode, BUT drag and drop still works in Retro mode (files only, not with folders).