That's indeed a bug... Not sure if our DIS team is able to reproduce this issue, but the login page should indeed include the Microsoft button. It's actually the same login page as you can find on dataminer.services.

It looks like this is some kind of scaling issue. It's like the login page is a lot bigger than what the hosting window expects it to be, and therefore the scrollbar is not going far enough. This probably has something to do with the resolution, scaling or DPI setting of the OS, or zoom level of the web browser. But I'll leave it to the DIS team to figure that out...