<!-- SPDX-License-Identifier: MIT -->
<!-- SPDX-FileCopyrightText: 2025-2026 Marcus Quinn -->

# Design System: Linear — Depth & Elevation

| Level | Treatment | Use |
|-------|-----------|-----|
| Flat (Level 0) | No shadow, `#010102` bg | Page background, deepest canvas |
| Subtle (Level 1) | `rgba(0,0,0,0.03) 0px 1.2px 0px` | Toolbar buttons, micro-elevation |
| Surface (Level 2) | `rgba(255,255,255,0.05)` bg + `1px solid rgba(255,255,255,0.08)` border | Cards, input fields, containers |
| Inset (Level 2b) | `rgba(0,0,0,0.2) 0px 0px 12px 0px inset` | Recessed panels, inner shadows |
| Ring (Level 3) | `rgba(0,0,0,0.2) 0px 0px 0px 1px` | Border-as-shadow technique |
| Elevated (Level 4) | `rgba(0,0,0,0.4) 0px 2px 4px` | Floating elements, dropdowns |
| Dialog (Level 5) | Multi-layer stack: `rgba(0,0,0,0) 0px 8px 2px, rgba(0,0,0,0.01) 0px 5px 2px, rgba(0,0,0,0.04) 0px 3px 2px, rgba(0,0,0,0.07) 0px 1px 1px, rgba(0,0,0,0.08) 0px 0px 1px` | Popovers, command palette, modals |
| Focus | `rgba(0,0,0,0.1) 0px 4px 12px` + additional layers | Keyboard focus on interactive elements |

**Shadow Philosophy**: On dark surfaces, traditional shadows (dark on dark) are nearly invisible. Linear solves this by using semi-transparent white borders as the primary depth indicator. Elevation isn't communicated through shadow darkness but through background luminance steps — each level slightly increases the white opacity of the surface background (`0.02` → `0.04` → `0.05`), creating a subtle stacking effect. The inset shadow technique (`rgba(0,0,0,0.2) 0px 0px 12px 0px inset`) creates a unique "sunken" effect for recessed panels, adding dimensional depth that traditional dark themes lack.
