Haiku Core is what our Mac app uses to render the stage. (Our app is built in Electron.) Here's the GitHub link if you're interested: https://github.com/HaikuTeam/core
Fair point! Short answer: Even though our UI is totally platform-agnostic, we have a lot going on under the hood with platform-specific considerations. Examples include our Sketch integration (which, like Sketch, will have to be Mac-exclusive), ensuring our CLI works with the various ways our users can install Node/npm/yarn and Git, and supporting various firewall/antivirus configs in conjunction with our live design features. Like we said, multi-platform support is coming soon — focusing on Mac just helped us ship sooner.