Scheduling and managing events using Outlook.com calendar
I don't think it was stable, or even true multitasking, until NT, which I think would have been Windows 9. And Microsoft couldn't do it: “Microsoft hired a group of developers from Digital Equipment Corporation led by Dave Cutler to build Windows NT…” https://en.m.wikipedia.org/wiki/Windows\_NT
Microsoft has never been interested in honing a product to perfection. Good enough is good enough, and it's on to the next big thing. In the last couple of years they probably spent way more on Aptos, the new font they forced on me, than on, say, giving office vba a decent editor.