fix(cdk/overlay): load structural styles in a cascade layer

Recently we switched to loading the styles of the overlay dynamically which can make them more specific than pre-existing themes. These changes use a cascade layer in order to reduce their specificity.

src/cdk/overlay/BUILD.bazel

@@ -19,7 +19,7 @@ ng_module(
         exclude = ["**/*.spec.ts"],
     ),
     assets = [
-        ":overlay-prebuilt.css",
+        ":overlay-structure.css",
     ],
     deps = [
         "//src:dev_mode_types",
@@ -46,7 +46,14 @@ sass_binary(
     src = "overlay-prebuilt.scss",
     deps = [
         ":overlay_scss_lib",
-        "//src/cdk/a11y:a11y_scss_lib",
+    ],
+)
+
+sass_binary(
+    name = "overlay_structure_scss",
+    src = "overlay-structure.scss",
+    deps = [
+        ":overlay_scss_lib",
     ],
 )
 

src/cdk/overlay/_index.scss

@@ -12,8 +12,20 @@ $overlay-backdrop-color: rgba(0, 0, 0, 0.32) !default;
 $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() {
+// Conditionally wraps some styles in a layer depending on a flag.
+@mixin _conditional-layer($should-wrap) {
+  @if ($should-wrap) {
+    @layer cdk-overlay {
+      @content;
+    }
+  } @else {
+    @content;
+  }
+}
+
+// Structural styles for the overlay. Pass `$wrap-customizable-styles` to emit
+// the styles that support customization in a way that makes them easier to change.
+@mixin private-overlay-structure($wrap-customizable-styles) {
   .cdk-overlay-container, .cdk-global-overlay-wrapper {
     // Disable events from being captured on the overlay container.
     pointer-events: none;
@@ -28,7 +40,10 @@ $backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;
   // The overlay-container is an invisible element which contains all individual overlays.
   .cdk-overlay-container {
     position: fixed;
-    z-index: $overlay-container-z-index;
+
+    @include _conditional-layer($wrap-customizable-styles) {
+      z-index: $overlay-container-z-index;
+    }
 
     &:empty {
       // Hide the element when it doesn't have any child nodes. This doesn't
@@ -44,7 +59,10 @@ $backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;
   .cdk-global-overlay-wrapper {
     display: flex;
     position: absolute;
-    z-index: $overlay-z-index;
+
+    @include _conditional-layer($wrap-customizable-styles) {
+      z-index: $overlay-z-index;
+    }
   }
 
   // A single overlay pane.
@@ -54,13 +72,16 @@ $backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;
     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%;
+
+    @include _conditional-layer($wrap-customizable-styles) {
+      z-index: $overlay-z-index;
+    }
   }
 
   .cdk-overlay-backdrop {
@@ -71,11 +92,14 @@ $backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;
     left: 0;
     right: 0;
 
-    z-index: $overlay-backdrop-z-index;
     pointer-events: auto;
     -webkit-tap-highlight-color: transparent;
-    transition: opacity $backdrop-animation-duration $backdrop-animation-timing-function;
     opacity: 0;
+
+    @include _conditional-layer($wrap-customizable-styles) {
+      z-index: $overlay-backdrop-z-index;
+      transition: opacity $backdrop-animation-duration $backdrop-animation-timing-function;
+    }
   }
 
   .cdk-overlay-backdrop-showing {
@@ -84,16 +108,17 @@ $backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;
     // 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 & {
+    @media (forced-colors: 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 {
-    // Add a CSS variable to make this easier to override.
-    background: var(--cdk-overlay-backdrop-dark-color, $overlay-backdrop-color);
+    @include _conditional-layer($wrap-customizable-styles) {
+      background: $overlay-backdrop-color;
+    }
   }
 
   .cdk-overlay-transparent-backdrop {
@@ -121,7 +146,6 @@ $backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;
   // 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
@@ -135,6 +159,10 @@ $backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;
     // Add some dimensions so the element has an `innerText` which some people depend on in tests.
     min-width: 1px;
     min-height: 1px;
+
+    @include _conditional-layer($wrap-customizable-styles) {
+      z-index: $overlay-z-index;
+    }
   }
 
   // Used when disabling global scrolling.
@@ -152,3 +180,8 @@ $backdrop-animation-timing-function: cubic-bezier(0.25, 0.8, 0.25, 1) !default;
     overflow-y: scroll;
   }
 }
+
+/// Emits structural styles required for cdk/overlay to function.
+@mixin overlay {
+  @include private-overlay-structure(false);
+}

src/cdk/overlay/overlay-container.ts

@@ -24,7 +24,7 @@ import {Platform, _isTestEnvironment} from '@angular/cdk/platform';
   changeDetection: ChangeDetectionStrategy.OnPush,
   encapsulation: ViewEncapsulation.None,
   standalone: true,
-  styleUrl: 'overlay-prebuilt.css',
+  styleUrl: 'overlay-structure.css',
   host: {'cdk-overlay-style-loader': ''},
 })
 export class _CdkOverlayStyleLoader {}

src/cdk/overlay/overlay-structure.scss

@@ -0,0 +1,3 @@
+@use './index' as overlay;
+
+@include overlay.private-overlay-structure(true);