The CSS Podcast: 019: Z-Index and Stacking Context

z-indexstacking contextThe CSS Podcast

1️⃣ Natural stacking order

  • Order of the element in the DOM.
  • Last child in DOM tree appears in the frontmost
  • CSS grid when assigning multiple elements into the same grid, without z-index, the order of stacking of the elements depends on their order in the DOM
  • flex-direction: reverse does not change the stacking order

2️⃣ z-index

  • can be positive / negative
  • need to set position other than static, in order to work
    • need a position to make z-index to have context
  • why z-index not working, position

3️⃣ negative z-index

  • negative z-index goes behind the parent.
  • html tag has base z-index, you can't go behind the <html>
<div class="parent">
  <!-- child hidden behind parent -->
  <div class="child"></div>
</div>
<!-- child still visible, can't hide behind html body -->
<div class="child"></div>

<style>
  div {
    width: 100px;
    height: 100px;
  }
  .child {
    position: absolute;
    z-index: -1;
    background: green;
  }
  .parent {
    background: red;
  }
  body {
    background: blue;
  }
</style>

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4️⃣ Highest z-index value

  • Not mentioned in the spec, limited by the implementation (32-bit signed integer)
  • (2^31)-1

5️⃣ Debugging z-index

  • Layers panel

    null

  • visbug /zindex plugin

    null

  • visbug /pesticide plugin

    null

visbug extension

6️⃣ Manage z-index

  • using css custom properties
  • name your z-index
:root {
  --dialog-z-index: 1;
  --popup-z-index: 2;
}

.my-popup {
  z-index: var(--popup-z-index);
}

.my-dialog {
  z-index: var(--dialog-z-index);
}

7️⃣ stacking context

  • html create the 1st stacking context
  • To create stacking context
    • position + z-index not auto
    • when creating composite layer - filters, opacity, transform, will-change
<div class="parent">
  <!-- z-index: 99 -->
  <div class="child1"></div>
</div>
<!-- z-index: 2 -->
<div class="child2"></div>

<style>
  div {
    width: 100px;
    height: 100px;
  }
  .parent {
    /* any of the following will create a new stacking context, */
    /* thus child2 will be on top of child1 */

    /* 1) position + z-index not auto */
    position: relative;
    z-index: 0;

    /*  2) filter  */
    filter: grayscale();

    /* 3) opacity < 1 */
    opacity: 0.9999999;

    /* 4) transform */
    transform: scale(1);

    /* 5) will change: transform, opacity, filter, z-index */
    will-change: opacity;
  }
  .child1 {
    background: green;
    position: absolute;
    top: 40px;
    left: 40px;
    z-index: 99;
  }
  .child2 {
    background: cyan;
    top: 20px;
    left: 20px;
    position: absolute;
    z-index: 2;
  }
</style>

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