The Invisible Details of Interaction Design

There's a moment in every great interface where you stop thinking about the interface itself. The button doesn't feel like a button—it feels like pressing something real. The animation doesn't feel like code—it feels like physics.

This is what Rauno Felber calls "invisible details of interaction design." The spring tension that makes a button feel satisfying to press. The momentum that makes scrolling feel connected to your finger. The way a modal appears not with a fade, but with the gentle elasticity of something being placed in the world.

Borrowed Metaphors

Great interaction design rewards learning by reusing metaphors. A swipe teaches you that the interface is layered like a deck of cards. Pinching maps to the precision of picking up tiny objects. These aren't decorative choices—they're borrowed from thousands of years of physical intuition.

The design engineer's job is to implement that intuition in code. Not to simulate physics perfectly, but to capture the feeling of physics. The essence, not the equations.

Spring Physics as Emotional Language

Consider two buttons: one that transitions with CSS ease-out, another with spring physics. The first feels digital. The second feels alive. Why? Because springs have personality. A tight spring feels nervous, energetic. A loose spring feels confident, relaxed.

// Nervous button - high stiffness, low damping

// Confident button - moderate stiffness, higher damping const confidentSpring = { stiffness: 200, damping: 25, mass: 1 } ```

This isn't just technical configuration—it's emotional configuration. The spring values become part of your design language, as important as your color palette or typography scale.

The Craft is in the Details

Linear's success isn't because it's a better project tracker. It's because every interaction feels considered. The way tasks slide into place. The gentle bounce when you complete something. The momentum when you scroll through a long list.

These details don't exist in Figma mockups. They emerge from design engineers who care about how things feel, not just how they look. Who understand that the gap between design and engineering isn't a problem to solve—it's where the craft lives.