I remember the exact moment the update landed on my phone in November 2014. Everything had changed overnight. The notification shade had depth now — a real z-axis, not just a translucent overlay. Alerts stacked like cards on a desk, each one a discrete sheet with a concrete shadow beneath it. The lock screen surfaced messages inline, without unlocking. It felt like the interface had been extruded from flat glass into something with actual volume.
Paper is not a metaphor
The design team did not deliver a cosmetic refresh. They built a spatial grammar for interactive surfaces. Every component gained elevation — a literal dp value that determined its shadow, stacking order, and relationship to the layer beneath. A floating action button at six dp hovered above a card at two dp, resting on the zero-dp baseline. The rules were simple and consistent: higher means closer to the user, more important, more immediate.