Hey, I work on Yammer, and I'm one of the contributors to this project. YamUI is a cross-team effort between our Design and Frontend teams to create drop-in components for Yammer.com that have strictly-enforced style and behavior. As @gregod said, it is based on Office's awesome Fabric project: https://github.com/OfficeDev/office-ui-fabric-react/
The project is just us working in the open. We're not trying to push this as a new component framework for others to use, and we probably don't have the bandwidth to support external consumers, but we're happy if people can learn from it.
Yup, that's because those hovercards render on `componentDidMount`, and the styleguide renders all components on page load. It's a bit janky, but it only affects the styleguide, so it's low priority.
Disclaimer: I work on Yammer, and I've contributed to this project.
It clusters the colors using Kohonen neural networks (which always makes me giggle, because "Kohonen" sounds like a Spanish swear word). The source for it all is here: https://github.com/unindented/palette-creator
Unrelated: Every week I receive an offer to "monetize" my extension from the shadiest companies all over the world. I'm guessing they'd add all kinds of tracking to it, make it inject ads and fake results into the pages the user is viewing, etc. So yeah, be careful who you trust when it comes to browser extensions!