Fluid Interfaces

Source: Apple WWDC 2018 session 803, "Designing Fluid Interfaces" (Chan Karunamuni, Nathan de Vries, Marcos Alonso — Human Interface team, built the iPhone X gestural UI). Full transcript in references/transcript.md.

Core thesis: an interface feels fluid when it feels like an extension of the user's mind and body, not a computer being operated. This isn't about frame rate — a 60fps interface can still feel "off." It comes from matching the interface's behavior to how our bodies and minds actually move: continuously, redirectably, and in response to real physical momentum.

Use this skill to audit existing gesture/motion code (find file:line violations of the principles below) or to design new interactions from scratch.

The 8 core principles