feat(theme): new CSS cascade layers plugin + built-in v4.useCssCascadeLayers future flag

packages/docusaurus-plugin-css-cascade-layers/.npmignore

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+.tsbuildinfo*
+tsconfig*
+__tests__

packages/docusaurus-plugin-css-cascade-layers/README.md

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+# `@docusaurus/plugin-css-cascade-layers`
+
+CSS Cascade Layer plugin for Docusaurus
+
+## Usage
+
+See [plugin-css-cascade-layers documentation](https://docusaurus.io/docs/api/plugins/@docusaurus/plugin-css-cascade-layers).

packages/docusaurus-plugin-css-cascade-layers/package.json

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+{
+  "name": "@docusaurus/plugin-css-cascade-layers",
+  "version": "3.7.0",
+  "description": "CSS Cascade Layer plugin for Docusaurus.",
+  "main": "lib/index.js",
+  "types": "lib/index.d.ts",
+  "scripts": {
+    "build": "tsc --build",
+    "watch": "tsc --build --watch"
+  },
+  "publishConfig": {
+    "access": "public"
+  },
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/facebook/docusaurus.git",
+    "directory": "packages/docusaurus-plugin-css-cascade-layers"
+  },
+  "license": "MIT",
+  "dependencies": {
+    "@docusaurus/core": "3.7.0",
+    "@docusaurus/types": "3.7.0",
+    "@docusaurus/utils-validation": "3.7.0",
+    "tslib": "^2.6.0"
+  },
+  "engines": {
+    "node": ">=18.0"
+  }
+}

packages/docusaurus-plugin-css-cascade-layers/src/__tests__/layers.test.ts

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+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {
+  generateLayersDeclaration,
+  findLayer,
+  isValidLayerName,
+} from '../layers';
+import type {PluginOptions} from '../options';
+
+describe('isValidLayerName', () => {
+  it('accepts valid names', () => {
+    expect(isValidLayerName('layer1')).toBe(true);
+    expect(isValidLayerName('layer1.layer2')).toBe(true);
+    expect(isValidLayerName('layer-1.layer_2.layer3')).toBe(true);
+  });
+
+  it('rejects layer with coma', () => {
+    expect(isValidLayerName('lay,er1')).toBe(false);
+  });
+  it('rejects layer with space', () => {
+    expect(isValidLayerName('lay er1')).toBe(false);
+  });
+});
+
+describe('generateLayersDeclaration', () => {
+  it('for list of layers', () => {
+    expect(generateLayersDeclaration(['layer1', 'layer2'])).toBe(
+      '@layer layer1, layer2;',
+    );
+  });
+
+  it('for empty list of layers', () => {
+    // Not useful to generate it, but still valid CSS anyway
+    expect(generateLayersDeclaration([])).toBe('@layer ;');
+  });
+});
+
+describe('findLayer', () => {
+  const inputFilePath = 'filePath';
+
+  function testFor(layers: PluginOptions['layers']) {
+    return findLayer(inputFilePath, Object.entries(layers));
+  }
+
+  it('for empty layers', () => {
+    expect(testFor({})).toBeUndefined();
+  });
+
+  it('for single matching layer', () => {
+    expect(testFor({layer: (filePath) => filePath === inputFilePath})).toBe(
+      'layer',
+    );
+  });
+
+  it('for single non-matching layer', () => {
+    expect(
+      testFor({layer: (filePath) => filePath !== inputFilePath}),
+    ).toBeUndefined();
+  });
+
+  it('for multiple matching layers', () => {
+    expect(
+      testFor({
+        layer1: (filePath) => filePath === inputFilePath,
+        layer2: (filePath) => filePath === inputFilePath,
+        layer3: (filePath) => filePath === inputFilePath,
+      }),
+    ).toBe('layer1');
+  });
+
+  it('for multiple non-matching layers', () => {
+    expect(
+      testFor({
+        layer1: (filePath) => filePath !== inputFilePath,
+        layer2: (filePath) => filePath !== inputFilePath,
+        layer3: (filePath) => filePath !== inputFilePath,
+      }),
+    ).toBeUndefined();
+  });
+
+  it('for multiple mixed matching layers', () => {
+    expect(
+      testFor({
+        layer1: (filePath) => filePath !== inputFilePath,
+        layer2: (filePath) => filePath === inputFilePath,
+        layer3: (filePath) => filePath !== inputFilePath,
+        layer4: (filePath) => filePath === inputFilePath,
+      }),
+    ).toBe('layer2');
+  });
+});

packages/docusaurus-plugin-css-cascade-layers/src/__tests__/options.test.ts

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+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {normalizePluginOptions} from '@docusaurus/utils-validation';
+import {
+  validateOptions,
+  type PluginOptions,
+  type Options,
+  DEFAULT_OPTIONS,
+} from '../options';
+import type {Validate} from '@docusaurus/types';
+
+function testValidateOptions(options: Options) {
+  return validateOptions({
+    validate: normalizePluginOptions as Validate<Options, PluginOptions>,
+    options,
+  });
+}
+
+describe('validateOptions', () => {
+  it('accepts undefined options', () => {
+    // @ts-expect-error: should error
+    expect(testValidateOptions(undefined)).toEqual(DEFAULT_OPTIONS);
+  });
+
+  it('accepts empty options', () => {
+    expect(testValidateOptions({})).toEqual(DEFAULT_OPTIONS);
+  });
+
+  describe('layers', () => {
+    it('accepts empty layers', () => {
+      expect(testValidateOptions({layers: {}})).toEqual({
+        ...DEFAULT_OPTIONS,
+        layers: {},
+      });
+    });
+
+    it('accepts undefined layers', () => {
+      const config: Options = {
+        layers: undefined,
+      };
+      expect(testValidateOptions(config)).toEqual(DEFAULT_OPTIONS);
+    });
+
+    it('accepts custom layers', () => {
+      const config: Options = {
+        layers: {
+          layer1: (filePath: string) => {
+            return !!filePath;
+          },
+          layer2: (filePath: string) => {
+            return !!filePath;
+          },
+        },
+      };
+      expect(testValidateOptions(config)).toEqual({
+        ...DEFAULT_OPTIONS,
+        layers: config.layers,
+      });
+    });
+
+    it('rejects layer with bad name', () => {
+      const config: Options = {
+        layers: {
+          'layer 1': (filePath) => !!filePath,
+        },
+      };
+      expect(() =>
+        testValidateOptions(config),
+      ).toThrowErrorMatchingInlineSnapshot(`""layers.layer 1" is not allowed"`);
+    });
+
+    it('rejects layer with bad value', () => {
+      const config: Options = {
+        layers: {
+          // @ts-expect-error: should error
+          layer1: 'bad value',
+        },
+      };
+      expect(() =>
+        testValidateOptions(config),
+      ).toThrowErrorMatchingInlineSnapshot(
+        `""layers.layer1" must be of type function"`,
+      );
+    });
+
+    it('rejects layer with bad function arity', () => {
+      const config: Options = {
+        layers: {
+          // @ts-expect-error: should error
+          layer1: () => {},
+        },
+      };
+      expect(() =>
+        testValidateOptions(config),
+      ).toThrowErrorMatchingInlineSnapshot(
+        `""layers.layer1" must have an arity of 1"`,
+      );
+    });
+  });
+});

packages/docusaurus-plugin-css-cascade-layers/src/index.ts

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+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import path from 'path';
+import {PostCssPluginWrapInLayer} from './postCssPlugin';
+import {generateLayersDeclaration} from './layers';
+import type {LoadContext, Plugin} from '@docusaurus/types';
+import type {PluginOptions, Options} from './options';
+
+const PluginName = 'docusaurus-plugin-css-cascade-layers';
+
+const LayersDeclarationModule = 'layers.css';
+
+function getLayersDeclarationPath(
+  context: LoadContext,
+  options: PluginOptions,
+) {
+  const {generatedFilesDir} = context;
+  const pluginId = options.id;
+  if (pluginId !== 'default') {
+    // Since it's only possible to declare a single layer order
+    // using this plugin twice doesn't really make sense
+    throw new Error(
+      'The CSS Cascade Layers plugin does not support multiple instances.',
+    );
+  }
+  return path.join(
+    generatedFilesDir,
+    PluginName,
+    pluginId,
+    LayersDeclarationModule,
+  );
+}
+
+export default function pluginCssCascadeLayers(
+  context: LoadContext,
+  options: PluginOptions,
+): Plugin | null {
+  const layersDeclarationPath = getLayersDeclarationPath(context, options);
+
+  return {
+    name: PluginName,
+
+    getClientModules() {
+      return [layersDeclarationPath];
+    },
+
+    async contentLoaded({actions}) {
+      await actions.createData(
+        LayersDeclarationModule,
+        generateLayersDeclaration(Object.keys(options.layers)),
+      );
+    },
+
+    configurePostCss(postCssOptions) {
+      postCssOptions.plugins.push(PostCssPluginWrapInLayer(options));
+      return postCssOptions;
+    },
+  };
+}
+
+export {validateOptions} from './options';
+
+export type {PluginOptions, Options};

packages/docusaurus-plugin-css-cascade-layers/src/layers.ts

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+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+export type LayerEntry = [string, (filePath: string) => boolean];
+
+export function isValidLayerName(layer: string): boolean {
+  // TODO improve validation rule to match spec, not high priority
+  return !layer.includes(',') && !layer.includes(' ');
+}
+
+export function generateLayersDeclaration(layers: string[]): string {
+  return `@layer ${layers.join(', ')};`;
+}
+
+export function findLayer(
+  filePath: string,
+  layers: LayerEntry[],
+): string | undefined {
+  // Using find() => layers order matter
+  // The first layer that matches is used in priority even if others match too
+  const layerEntry = layers.find((layer) => layer[1](filePath));
+  return layerEntry?.[0]; // return layer name
+}