[CSS design] I think about specifying z-index difficult to break

Summary

Defines the z-index value to be specified for each component.

Motivation

• I want to avoid z-index conflicts
• Want to clarify the reason for specifying z-index
• Don't want to increase z-index each time more components are added

Prerequisites

• The stacking context - MDN
• isolation - MDN

Rules for z-index

• Do not use z-index as much as possible.
• Use z-index from 0 to 4.
• As a rule, do not use negative numbers such as -1.

Layer Stack rules

• Define the z-index value that a component should have on a layer (hierarchy) for each role.
• Increase the z-index value as the layer stacks up.

Layer Stack 4 : z-index: 4.

Element to be placed on top in any state.

• Modal
• Drawer
• Overlay

Layer Stack 3 : z-index: 3

Elements that are always placed on top.

An element that is always placed on top.

• Global header (e.g., fixed header to follow)

Layer Stack 2 : z-index: 2.

• Tooltips
• Pop-overs
• Floating buttons (e.g., arrow buttons for carousels, etc.)

Layer Stack 1 : z-index: 1.

Placed on top of an element that does not have a stack.

• Badge

Define z-index.

:root {
  --layer-stack-1: 1;
  --layer-stack-2: 2;
  --layer-stack-3: 3;
  --layer-stack-4: 4;
}

.modal {
  z-index: var(--layer-stack-4);
}

.modal-in-modal {
  z-index: calc(var(--layer-stack-4) + 1);
}

Rules for isolation

There may be a case where the z-index conflicts between components in the Layer Stack definition alone, resulting in unintended overlapping. In such a case, you can solve the problem by using isolation to generate stacking context in the root of the component.

.special-components {
  isolation: isolate; /* generate stacking context */
  position: absolute;
}

.special-components__item {
  position: absolute;
  z-index: 100;
}

.modal {
  position: absolute;
  z-index: var(--layer-stack-4);
}

Basically, it is preferable to use an HTML structure with no problems with just a z-index specification, but there is a high possibility of unintended overlap with other components when a specification such as z-index: calc(var(--layer-stack-2) + 1); is used. It is preferable to specify isolation for the root of a component whose child elements have a z-index specification.

Incidentally, before the advent of isolation, there was a hack to generate stacking contexts by specifying transform without side-effects (e.g. transform: scale(1)).

Summary

• Specify z-index according to Layer Stack rules.
• Specify isolation: isolate for component root