Z-Index Scale

Overview

Generate a systematic z-index token scale that eliminates arbitrary values and z-index conflicts. Creates predictable layering for dropdowns, modals, tooltips, and overlays.

When to Use

• Setting up z-index tokens for a new design system
• Fixing z-index conflicts and "wars" in existing code
• Creating consistent layering for overlays, modals, tooltips
• Documenting stacking context expectations

Quick Reference: Layering Levels

The Process

1. Understand stacking needs: Identify all layered components in the system
2. Ask output format: CSS custom properties, Tailwind config, or JSON tokens
3. Generate scale: Create tokens with semantic naming
4. Document usage: Explain which token for which use case

Output Formats

CSS Custom Properties

:root {
  /* Base - default document flow */
  --z-index-base: 0;
  --z-index-below: -1;

  /* Raised - subtle elevation within content */
  --z-index-raised: 1;
  --z-index-raised-2: 2;
  --z-index-raised-3: 3;

  /* Dropdown - menus, selects, comboboxes, popovers */
  --z-index-dropdown: 100;

  /* Sticky - sticky positioned elements */
  --z-index-sticky: 200;

  /* Fixed - fixed navigation, floating action buttons */
  --z-index-fixed: 300;
  --z-index-header: 310;
  --z-index-sidebar: 320;

  /* Overlay - backdrop behind modals */
  --z-index-overlay: 400;

  /* Modal - dialogs, sheets, drawers */
  --z-index-modal: 500;
  --z-index-modal-content: 510;

  /* Toast - notifications, snackbars */
  --z-index-toast: 600;

  /* Tooltip - tooltips, title attributes */
  --z-index-tooltip: 700;

  /* Maximum - debug, dev tools (avoid in production) */
  --z-index-max: 9999;
}

Tailwind Config

// tailwind.config.js
module.exports = {
  theme: {
    extend: {
      zIndex: {
        'below': '-1',
        'base': '0',
        'raised': '1',
        'raised-2': '2',
        'raised-3': '3',
        'dropdown': '100',
        'sticky': '200',
        'fixed': '300',
        'header': '310',
        'sidebar': '320',
        'overlay': '400',
        'modal': '500',
        'modal-content': '510',
        'toast': '600',
        'tooltip': '700',
        'max': '9999',
      },
    },
  },
};

Usage:

<div class="z-overlay">Backdrop</div>
<div class="z-modal">Modal content</div>
<div class="z-tooltip">Tooltip</div>

JSON Tokens (Design Tokens Format)

{
  "z-index": {
    "below": { "value": -1, "description": "Below base content" },
    "base": { "value": 0, "description": "Default document flow" },
    "raised": {
      "1": { "value": 1, "description": "Subtle elevation" },
      "2": { "value": 2, "description": "Medium elevation" },
      "3": { "value": 3, "description": "High elevation" }
    },
    "dropdown": { "value": 100, "description": "Menus, selects, popovers" },
    "sticky": { "value": 200, "description": "Sticky positioned elements" },
    "fixed": {
      "base": { "value": 300, "description": "Fixed elements" },
      "header": { "value": 310, "description": "Fixed header" },
      "sidebar": { "value": 320, "description": "Fixed sidebar" }
    },
    "overlay": { "value": 400, "description": "Backdrop behind modals" },
    "modal": {
      "base": { "value": 500, "description": "Modal container" },
      "content": { "value": 510, "description": "Modal inner content" }
    },
    "toast": { "value": 600, "description": "Notifications, snackbars" },
    "tooltip": { "value": 700, "description": "Tooltips" },
    "max": { "value": 9999, "description": "Maximum (debug only)" }
  }
}

Stacking Context Concepts

What Creates a New Stacking Context

Understanding stacking contexts prevents unexpected z-index behavior:

/* These properties create new stacking contexts */
.creates-context {
  /* Any of these: */
  position: relative; z-index: 1; /* positioned + z-index */
  position: fixed;                 /* fixed positioning */
  position: sticky;                /* sticky positioning */
  opacity: 0.99;                   /* opacity < 1 */
  transform: translateZ(0);        /* any transform */
  filter: blur(0);                 /* any filter */
  isolation: isolate;              /* explicit isolation */
  contain: layout;                 /* containment */
  will-change: transform;          /* will-change hint */
}

Isolation Pattern

Use isolation: isolate to contain z-index within a component:

/* Component creates its own stacking context */
.card {
  isolation: isolate;
}

/* Internal z-index stays within the card */
.card__overlay {
  z-index: 1; /* Only relative to .card, not page */
}

.card__content {
  z-index: 2;
}

Component Patterns

Modal with Backdrop

.modal-backdrop {
  position: fixed;
  inset: 0;
  z-index: var(--z-index-overlay);
  background: rgb(0 0 0 / 0.5);
}

.modal {
  position: fixed;
  z-index: var(--z-index-modal);
  /* centering styles */
}

/* Nested modal (rare but possible) */
.modal--nested {
  z-index: calc(var(--z-index-modal) + 10);
}

Dropdown in Fixed Header

.header {
  position: fixed;
  top: 0;
  z-index: var(--z-index-header);
  isolation: isolate; /* Contain dropdown z-index */
}

.header__dropdown {
  position: absolute;
  z-index: var(--z-index-dropdown);
}

Toast Notifications

.toast-container {
  position: fixed;
  bottom: var(--spacing-lg);
  right: var(--spacing-lg);
  z-index: var(--z-index-toast);

  /* Stack toasts */
  display: flex;
  flex-direction: column-reverse;
  gap: var(--spacing-sm);
}

Tooltip

.tooltip {
  position: absolute;
  z-index: var(--z-index-tooltip);
  /* Always on top, even over modals */
}

Common Patterns

Progressive Scale

For simple projects, use powers of 10:

:root {
  --z-dropdown: 10;
  --z-sticky: 20;
  --z-fixed: 30;
  --z-modal: 40;
  --z-toast: 50;
  --z-tooltip: 60;
}

Hundred-Based Scale

For complex apps needing sub-levels:

:root {
  --z-dropdown: 100;
  --z-dropdown-nested: 110;
  --z-sticky: 200;
  --z-modal: 500;
  --z-modal-nested: 510;
}

Named Layers (CSS Cascade Layers)

Modern alternative using @layer:

@layer base, components, overlays, modals, notifications;

@layer overlays {
  .dropdown { /* Automatically layered */ }
}

@layer modals {
  .modal { /* Higher than overlays */ }
}

Anti-Patterns to Avoid

Debugging Z-Index Issues

Browser DevTools

1. Elements panel → Computed → search "z-index"
2. 3D View (Chrome) → Shows stacking visually
3. Check if parent creates stacking context

Debug Utility

/* Temporarily visualize z-index layers */
[style*="z-index"] {
  outline: 2px solid red !important;
}

[style*="z-index"]::after {
  content: attr(style);
  position: absolute;
  background: red;
  color: white;
  font-size: 10px;
  padding: 2px;
}

Common Fixes

/* Problem: z-index not working */
.element {
  z-index: 100; /* Won't work without positioning */
}

/* Fix: Add positioning */
.element {
  position: relative; /* or absolute/fixed */
  z-index: 100;
}

/* Problem: Child higher than parent's sibling */
/* Fix: Use isolation on parent */
.parent {
  isolation: isolate;
}

Integration with Other Tokens

:root {
  /* Z-index works with shadows for elevation */
  --elevation-1: var(--z-index-raised), var(--shadow-sm);
  --elevation-2: var(--z-index-dropdown), var(--shadow-md);
  --elevation-3: var(--z-index-modal), var(--shadow-lg);
}

/* Usage */
.card {
  z-index: var(--z-index-raised);
  box-shadow: var(--shadow-sm);
}