feat: add helper functions and docs

Adds helper functions to simplify working with `merge-styles`
shadow DOM APIs.

Adds documentation explaining basic usage of shadow DOM features.

Author: spmonahan

packages/merge-styles/README.md

@@ -504,3 +504,76 @@ window.FabricConfig = {
   },
 };
 ```
+
+## Shadow DOM
+
+`merge-styles` has experimental support for [shadow DOM](https://developer.mozilla.org/en-US/docs/Web/API/Web_components/Using_shadow_DOM). This feature is opt-in and incrementally adoptable. To enable the feature you need to include two [React Providers](https://react.dev/reference/react/createContext#provider):
+
+1. `MergeStylesRootProvider`: acts as the "global" context for your application. You should have one of these per page.
+2. `MergeStylesShadowRootProvider`: a context for each shadow root in your application. You should have one of these per shadow root.
+
+`merge-styles` does not provide an option for creating shadow roots in React as how you get a shadow root doesn't matter, just that you have a reference to one. [`react-shadow`](https://www.npmjs.com/package/react-shadow) is one library that can create shadow roots in React and will be used in examples.
+
+### Shadow DOM example
+
+```tsx
+import { PrimaryButton } from '@fluentui/react';
+import { MergeStylesRootProvider, MergeStylesShadowRootProvider } from '@fluentui/utilities';
+import root from 'react-shadow';
+
+const ShadowRoot = ({ children }) => {
+  // This is a ref but we're using state to manage it so we can force
+  // a re-render.
+  const [shadowRootEl, setShadowRootEl] = React.useState<HTMLElement | null>(null);
+
+  return (
+    <MergeStylesRootProvider>
+      <root.div className="shadow-root" delegatesFocus ref={setShadowRootEl}>
+        <MergeStylesShadowRootProvider shadowRoot={shadowRootEl?.shadowRoot}>{children}</MergeStylesShadowRootProvider>
+      </root.div>
+    </MergeStylesRootProvider>
+  );
+};
+
+<ShadowRoot>
+  <PrimaryButton>I'm in the shadow DOM!</PrimaryButton>
+</ShadowRoot>
+<PrimaryButton>I'm in the light DOM!</PrimaryButton>
+```
+
+### Scoping styles for more efficient CSS
+
+You do not _need_ to update your `merge-styles` styles to support shadow DOM but you can make styles more efficient with some updates.
+
+Shadow DOM support is achieved in `merge-styles` by using [constructable stylesheets](https://web.dev/articles/constructable-stylesheets) and is scoped by "stylesheet keys". `merge-styles` creates one stylesheet per key and in Fluent this means each component has its own stylesheet. Each `MergeStylesShadowRootProvider` will only adopt styles for components it contains plus the global sheet (we cannot be certain whether we need this sheet or not so we always adopt it). This means a `MergeStylesShadowRootProvider` that contains a button will only adopt button styles (plus the global styles) but not checkbox styles, making styling within the shadow root more efficient.
+
+If you use `customizable` or `styled` the existing "scope" value provided to these functions is used a unique key. If no key is provided `merge-styles` falls back to a "global" key. This global key is a catch-all and allows us to support code that was written before shadow DOM support was added or code that is called outside of React context.
+
+All `@fluentui/react` styles are scoped to via `customizable` and `styled` (and some updates to specific component styles where needed). If your components use these functions and you set the "scope" property your components will automatically be scoped.
+If you're using `mergeStyles()` (and other `merge-styles` APIs) directly, your styles will be placed in the global scope and still be available in shadow roots, just not as optimally as possible.
+
+#### Style scoping example
+
+```tsx
+import { useWindow } from '@fluentui/react';
+import { mergeStyles } from '@fluentui/merge-styles';
+import { useShadowConfig } from '@fluentui/utilities';
+import type { ShadowConfig } from '@fluentui/merge-styles';
+
+// This must be globally unique for the application
+const MY_COMPONENT_STYLESHEET_KEY: string = 'my-unique-key';
+
+const MyComponent = props => {
+  // Make sure multi-window scenarios work (e.g., pop outs)
+  const win: Window = useWindow();
+  // Check if the component is rendered in a shadow context
+  const inShadow: boolean = useHasMergeStylesShadowRootContext();
+
+  const shadowConfig: ShadowConfig = useShadowConfig(MY_COMPONENT_STYLESHEET_KEY, win);
+
+  const styles = React.useMemo(() => {
+    // shadowConfig must be the first parameter when it is used
+    return mergeStyles(shadowConfig, myStyles);
+  }, [shadowConfig, myStyles]);
+};
+```

packages/merge-styles/etc/merge-styles.api.md

@@ -469,6 +469,9 @@ export interface IStyleSheetConfig {
 // @public
 export function keyframes(timeline: IKeyframes): string;
 
+// @public (undocumented)
+export const makeShadowConfig: (stylesheetKey: string, inShadow: boolean, window?: Window) => ShadowConfig;
+
 // Warning: (ae-forgotten-export) The symbol "IStyleOptions" needs to be exported by the entry point index.d.ts
 //
 // @public

packages/merge-styles/src/index.ts

@@ -39,7 +39,7 @@ export { setRTL } from './StyleOptionsState';
 
 export type { ObjectOnly } from './ObjectOnly';
 
-export { GLOBAL_STYLESHEET_KEY } from './shadowConfig';
+export { GLOBAL_STYLESHEET_KEY, makeShadowConfig } from './shadowConfig';
 export type { ShadowConfig } from './shadowConfig';
 
 import './version';

packages/merge-styles/src/shadowConfig.ts

@@ -14,6 +14,15 @@ export const DEFAULT_SHADOW_CONFIG: ShadowConfig = {
   __isShadowConfig__: true,
 } as const;
 
+export const makeShadowConfig = (stylesheetKey: string, inShadow: boolean, window?: Window): ShadowConfig => {
+  return {
+    stylesheetKey,
+    inShadow,
+    window,
+    __isShadowConfig__: true,
+  };
+};
+
 export const isShadowConfig = (obj: unknown): obj is ShadowConfig => {
   if (!obj) {
     return false;

packages/utilities/src/customizations/customizable.tsx

@@ -2,7 +2,7 @@ import * as React from 'react';
 import { Customizations } from './Customizations';
 import { hoistStatics } from '../hoistStatics';
 import { CustomizerContext } from './CustomizerContext';
-import { concatStyleSets } from '@fluentui/merge-styles';
+import { concatStyleSets, makeShadowConfig } from '@fluentui/merge-styles';
 import type { ICustomizerContext } from './CustomizerContext';
 import { MergeStylesShadowRootConsumer } from '../shadowDom/MergeStylesShadowRootContext';
 import type { ShadowConfig } from '@fluentui/merge-styles';
@@ -56,14 +56,8 @@ export function customizable(
                       this._shadowConfig.stylesheetKey !== scope ||
                       this._shadowConfig.inShadow !== inShadow ||
                       this._shadowConfig.window !== win
-                      // false
                     ) {
-                      this._shadowConfig = {
-                        stylesheetKey: scope,
-                        inShadow,
-                        window: win,
-                        __isShadowConfig__: true,
-                      };
+                      this._shadowConfig = makeShadowConfig(scope, inShadow, win);
                     }
 
                     // eslint-disable-next-line @typescript-eslint/no-explicit-any

packages/utilities/src/shadowDom/MergeStylesRootContext.tsx

@@ -1,5 +1,5 @@
 import * as React from 'react';
-import { GLOBAL_STYLESHEET_KEY, Stylesheet } from '@fluentui/merge-styles';
+import { GLOBAL_STYLESHEET_KEY, Stylesheet, makeShadowConfig } from '@fluentui/merge-styles';
 import { getWindow } from '../dom';
 
 declare global {
@@ -82,12 +82,14 @@ export const MergeStylesRootProvider: React.FC<MergeStylesRootProviderProps> = (
 
     let changed = false;
     const next = new Map<string, CSSStyleSheet>(stylesheets);
-    const sheet = Stylesheet.getInstance({
-      window: win,
-      inShadow: false,
-      stylesheetKey: GLOBAL_STYLESHEET_KEY,
-      __isShadowConfig__: true,
-    });
+    const sheet = Stylesheet.getInstance(makeShadowConfig(GLOBAL_STYLESHEET_KEY, false, win));
+
+    //   {
+    //   window: win,
+    //   inShadow: false,
+    //   stylesheetKey: GLOBAL_STYLESHEET_KEY,
+    //   __isShadowConfig__: true,
+    // });
     sheet.forEachAdoptedStyleSheet((adoptedSheet, key) => {
       next.set(key, adoptedSheet);
       changed = true;

packages/utilities/src/shadowDom/MergeStylesShadowRootContext.tsx

@@ -1,5 +1,5 @@
 import * as React from 'react';
-import { GLOBAL_STYLESHEET_KEY } from '@fluentui/merge-styles';
+import { GLOBAL_STYLESHEET_KEY, makeShadowConfig } from '@fluentui/merge-styles';
 import { FocusRectsProvider } from '../FocusRectsProvider';
 import { useMergeStylesRootStylesheets } from './MergeStylesRootContext';
 
@@ -112,3 +112,16 @@ export const useHasMergeStylesShadowRootContext = () => {
 export const useMergeStylesShadowRootContext = () => {
   return React.useContext(MergeStylesShadowRootContext);
 };
+
+/**
+ * Get a shadow config.
+ * @param stylesheetKey - Globally unique key
+ * @param win - Reference to the `window` global.
+ * @returns ShadowConfig
+ */
+export const useShadowConfig = (stylesheetKey: string, win?: Window) => {
+  const inShadow = useHasMergeStylesShadowRootContext();
+  return React.useMemo(() => {
+    return makeShadowConfig(stylesheetKey, inShadow, win);
+  }, [stylesheetKey, inShadow, win]);
+};

packages/utilities/src/styled.tsx

@@ -1,6 +1,6 @@
 import * as React from 'react';
 import { concatStyleSetsWithProps } from '@fluentui/merge-styles';
-import { useAdoptedStylesheet, useHasMergeStylesShadowRootContext } from './shadowDom/MergeStylesShadowRootContext';
+import { useAdoptedStylesheet, useShadowConfig } from './shadowDom/MergeStylesShadowRootContext';
 import { useCustomizationSettings } from './customizations/useCustomizationSettings';
 import type { IStyleSet, IStyleFunctionOrObject, ShadowConfig } from '@fluentui/merge-styles';
 import { getWindow } from './dom/getWindow';
@@ -103,21 +103,7 @@ export function styled<
     const additionalProps = getProps ? getProps(props) : undefined;
 
     const win = useWindow() ?? getWindow();
-
-    const inShadow = useHasMergeStylesShadowRootContext();
-    const shadowConfig = React.useRef<ShadowConfig>({ stylesheetKey: scope, inShadow, __isShadowConfig__: true });
-    if (
-      shadowConfig.current.stylesheetKey !== scope ||
-      shadowConfig.current.inShadow !== inShadow ||
-      shadowConfig.current.window !== win
-    ) {
-      shadowConfig.current = {
-        stylesheetKey: scope,
-        inShadow,
-        window: win,
-        __isShadowConfig__: true,
-      };
-    }
+    const shadowConfig = useShadowConfig(scope, win);
 
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const cache = (styles.current && (styles.current as any).__cachedInputs__) || [];
@@ -143,7 +129,7 @@ export function styled<
       styles.current = concatenatedStyles as StyleFunction<TStyleProps, TStyleSet>;
     }
 
-    styles.current.__shadowConfig__ = shadowConfig.current;
+    styles.current.__shadowConfig__ = shadowConfig;
 
     useAdoptedStylesheet(scope);