fix(material/core): M3 themes not inserting loaded marker

Fixes that all M3 themes were causing a "no theme has been loaded" warning to be logged, because they weren't inserting the loaded marker.

Note: it's tempting to create the marker as a token, but we can't do it because tokens are output under a selector, whereas we want the marker to always be at the top level since we detect it by creating an element and inserting it into the `body`.

Fixes #29115.

Author: crisbeto

src/material/core/_core-theme.scss

@@ -16,6 +16,22 @@
 @use './tokens/m2/mat/full-pseudo-checkbox' as tokens-mat-full-pseudo-checkbox;
 @use './tokens/m2/mat/minimal-pseudo-checkbox' as tokens-mat-minimal-pseudo-checkbox;
 
+$_has-inserted-loaded-marker: false;
+
+@mixin _theme-loaded-marker {
+  @if not $_has-inserted-loaded-marker {
+    $_has-inserted-loaded-marker: true !global;
+
+    // Marker that is used to determine whether the user has added a theme to their page.
+    // Needs to be generated at the root, because themes may be nested inside classes.
+    @at-root {
+      .mat-theme-loaded-marker {
+        display: none;
+      }
+    }
+  }
+}
+
 @mixin base($theme) {
   @if inspection.get-theme-version($theme) == 1 {
     @include _theme-from-tokens(inspection.get-theme-tokens($theme, base));
@@ -26,6 +42,9 @@
     @include optgroup-theme.base($theme);
     @include pseudo-checkbox-theme.base($theme);
   }
+
+  // The marker is a concrete style no matter which Material version we're targeting.
+  @include _theme-loaded-marker;
 }
 
 @mixin color($theme) {
@@ -54,14 +73,6 @@
       }
     }
   }
-
-  // TODO(crisbeto): move this into the base.
-  // Marker that is used to determine whether the user has added a theme to their page.
-  @at-root {
-    .mat-theme-loaded-marker {
-      display: none;
-    }
-  }
 }
 
 @mixin typography($theme) {
@@ -127,6 +138,9 @@
       }
     }
   }
+
+  // The marker is a concrete style no matter which Material version we're targeting.
+  @include _theme-loaded-marker;
 }
 
 @mixin _theme-from-tokens($tokens, $options...) {

src/material/core/theming/tests/m3-theme.spec.ts

@@ -1,4 +1,4 @@
-import {parse} from 'postcss';
+import {parse, Rule} from 'postcss';
 import {compileString} from 'sass';
 import {runfiles} from '@bazel/runfiles';
 import * as path from 'path';
@@ -68,14 +68,18 @@ describe('M3 theme', () => {
     root.walkRules(rule => {
       selectors.push(rule.selector);
     });
-    expect(selectors).toEqual(['html']);
+    expect(selectors).toEqual(['html', '.mat-theme-loaded-marker']);
   });
 
   it('should only emit CSS variables', () => {
     const root = parse(transpile(`html { @include mat.all-component-themes($theme); }`));
     const nonVarProps: string[] = [];
     root.walkDecls(decl => {
-      if (!decl.prop.startsWith('--')) {
+      if (
+        !decl.prop.startsWith('--') &&
+        // Skip the theme loaded marker since it can't be a variable.
+        (decl.parent as Rule).selector !== '.mat-theme-loaded-marker'
+      ) {
         nonVarProps.push(decl.prop);
       }
     });

src/material/core/theming/tests/theming-inspection-api.spec.ts

@@ -472,7 +472,14 @@ describe('theming inspection api', () => {
         div {
           @include mat.all-component-themes($theme);
         }`);
-      expect(css).toBe('');
+      expect(css).toBe(
+        [
+          // The marker is always included.
+          `.mat-theme-loaded-marker {`,
+          `  display: none;`,
+          `}`,
+        ].join('\n'),
+      );
     });
   });
 });