@angular/cdk/overlay/_index.scss

// We want overlays to always appear over user content, so set a baseline
// very high z-index for the overlay container, which is where we create the new
// stacking context for all overlays.
$overlay-container-z-index: 1000 !default;
$overlay-z-index: 1000 !default;
$overlay-backdrop-z-index: 1000 !default;

// Background color for all of the backdrops
$overlay-backdrop-color: rgba(0, 0, 0, 0.32) !default;

// Default backdrop animation is based on the Material Design swift-ease-out.
$backdrop-animation-duration: 400ms !default;
$backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;

/// Emits structural styles required for cdk/overlay to function.
@mixin overlay() {
  .cdk-overlay-container, .cdk-global-overlay-wrapper {
    // Disable events from being captured on the overlay container.
    pointer-events: none;

    // The container should be the size of the viewport.
    top: 0;
    left: 0;
    height: 100%;
    width: 100%;
  }

  // The overlay-container is an invisible element which contains all individual overlays.
  .cdk-overlay-container {
    position: fixed;
    z-index: $overlay-container-z-index;

    &:empty {
      // Hide the element when it doesn't have any child nodes. This doesn't
      // include overlays that have been detached, rather than disposed.
      display: none;
    }
  }

  // We use an extra wrapper element in order to use make the overlay itself a flex item.
  // This makes centering the overlay easy without running into the subpixel rendering
  // problems tied to using `transform` and without interfering with the other position
  // strategies.
  .cdk-global-overlay-wrapper {
    display: flex;
    position: absolute;
    z-index: $overlay-z-index;
  }

  // A single overlay pane.
  .cdk-overlay-pane {
    // Note: it's important for this one to start off `absolute`,
    // in order for us to be able to measure it correctly.
    position: absolute;
    pointer-events: auto;
    box-sizing: border-box;
    z-index: $overlay-z-index;

    // For connected-position overlays, we set `display: flex` in
    // order to force `max-width` and `max-height` to take effect.
    display: flex;
    max-width: 100%;
    max-height: 100%;
  }

  .cdk-overlay-backdrop {
    // TODO(jelbourn): reuse sidenav fullscreen mixin.
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;

    z-index: $overlay-backdrop-z-index;
    pointer-events: auto;
    -webkit-tap-highlight-color: transparent;
    transition: opacity $backdrop-animation-duration $backdrop-animation-timing-function;
    opacity: 0;

    &.cdk-overlay-backdrop-showing {
      opacity: 1;

      // Note that we can't import and use the `high-contrast` mixin from `_a11y.scss`, because
      // this file will be copied to the top-level `cdk` package when putting together the files
      // for npm. Any relative import paths we use here will become invalid once the file is copied.
      .cdk-high-contrast-active & {
        // In high contrast mode the rgba background will become solid
        // so we need to fall back to making it opaque using `opacity`.
        opacity: 0.6;
      }
    }
  }

  .cdk-overlay-dark-backdrop {
    background: $overlay-backdrop-color;
  }

  .cdk-overlay-transparent-backdrop {
    // Define a transition on the visibility so that the `transitionend` event can fire immediately.
    transition: visibility 1ms linear, opacity 1ms linear;
    visibility: hidden;
    opacity: 1;

    // Note: as of Firefox 57, having the backdrop be `background: none` will prevent it from
    // capturing the user's mouse scroll events. Since we also can't use something like
    // `rgba(0, 0, 0, 0)`, we work around the inconsistency by not setting the background at
    // all and using `opacity` to make the element transparent.
    &.cdk-overlay-backdrop-showing {
      opacity: 0;
      visibility: visible;
    }
  }

  .cdk-overlay-backdrop-noop-animation {
    transition: none;
  }

  // Overlay parent element used with the connected position strategy. Used to constrain the
  // overlay element's size to fit within the viewport.
  .cdk-overlay-connected-position-bounding-box {
    position: absolute;
    z-index: $overlay-z-index;

    // We use `display: flex` on this element exclusively for centering connected overlays.
    // When *not* centering, a top/left/bottom/right will be set which overrides the normal
    // flex layout.
    display: flex;

    // We use the `column` direction here to avoid some flexbox issues in Edge
    // when using the "grow after open" options.
    flex-direction: column;

    // Add some dimensions so the element has an `innerText` which some people depend on in tests.
    min-width: 1px;
    min-height: 1px;
  }

  // Used when disabling global scrolling.
  .cdk-global-scrollblock {
    position: fixed;

    // Necessary for the content not to lose its width. Note that we're using 100%, instead of
    // 100vw, because 100vw includes the width plus the scrollbar, whereas 100% is the width
    // that the element had before we made it `fixed`.
    width: 100%;

    // Note: this will always add a scrollbar to whatever element it is on, which can
    // potentially result in double scrollbars. It shouldn't be an issue, because we won't
    // block scrolling on a page that doesn't have a scrollbar in the first place.
    overflow-y: scroll;
  }
}