feat: add flag for forced shadow migrate mode

packages/@lwc/engine-core/src/framework/base-lightning-element.ts

@@ -20,9 +20,9 @@ import {
     freeze,
     hasOwnProperty,
     isFunction,
-    isString,
     isNull,
     isObject,
+    isString,
     isUndefined,
     KEY__SYNTHETIC_MODE,
     keys,
@@ -35,7 +35,7 @@ import { applyAriaReflection } from '../libs/aria-reflection/aria-reflection';
 
 import { HTMLElementOriginalDescriptors } from './html-properties';
 import { getWrappedComponentsListener } from './component';
-import { vmBeingConstructed, isBeingConstructed, isInvokingRender } from './invoker';
+import { isBeingConstructed, isInvokingRender, vmBeingConstructed } from './invoker';
 import {
     associateVM,
     getAssociatedVM,
@@ -47,16 +47,17 @@ import {
 } from './vm';
 import { componentValueObserved } from './mutation-tracker';
 import {
-    patchShadowRootWithRestrictions,
-    patchLightningElementPrototypeWithRestrictions,
     patchCustomElementWithRestrictions,
+    patchLightningElementPrototypeWithRestrictions,
+    patchShadowRootWithRestrictions,
 } from './restrictions';
-import { Template, isUpdatingTemplate, getVMBeingRendered } from './template';
+import { getVMBeingRendered, isUpdatingTemplate, Template } from './template';
 import { HTMLElementConstructor } from './base-bridge-element';
 import { updateComponentValue } from './update-component-value';
 import { markLockerLiveObject } from './membrane';
 import { TemplateStylesheetFactories } from './stylesheet';
 import { instrumentInstance } from './runtime-instrumentation';
+import { applyShadowMigrateMode } from './shadow-migration-mode';
 
 /**
  * This operation is called with a descriptor of an standard html property
@@ -289,6 +290,14 @@ function doAttachShadow(vm: VM): ShadowRoot {
         patchShadowRootWithRestrictions(shadowRoot);
     }
 
+    if (
+        process.env.IS_BROWSER &&
+        lwcRuntimeFlags.ENABLE_FORCE_SHADOW_MIGRATE_MODE &&
+        vm.shadowMigrateMode
+    ) {
+        applyShadowMigrateMode(shadowRoot);
+    }
+
     return shadowRoot;
 }
 

packages/@lwc/engine-core/src/framework/shadow-migration-mode.ts

@@ -0,0 +1,84 @@
+/*
+ * Copyright (c) 2023, salesforce.com, inc.
+ * All rights reserved.
+ * SPDX-License-Identifier: MIT
+ * For full license text, see the LICENSE file in the repo root or https://opensource.org/licenses/MIT
+ */
+import { logWarnOnce } from '../shared/logger';
+
+let globalStylesheet: CSSStyleSheet | undefined;
+
+function isStyleElement(elm: Element): elm is HTMLStyleElement {
+    return elm.tagName === 'STYLE';
+}
+
+async function fetchStylesheet(elm: HTMLStyleElement | HTMLLinkElement) {
+    if (isStyleElement(elm)) {
+        return elm.textContent;
+    } else {
+        // <link>
+        const { href } = elm;
+        try {
+            return await (await fetch(href)).text();
+        } catch (err) {
+            logWarnOnce(`Ignoring cross-origin stylesheet in migrate mode: ${href}`);
+            // ignore errors with cross-origin stylesheets - nothing we can do for those
+            return '';
+        }
+    }
+}
+
+function initGlobalStylesheet() {
+    const stylesheet = new CSSStyleSheet();
+    const elmsToPromises = new Map();
+    let lastSeenLength = 0;
+
+    const copyToGlobalStylesheet = () => {
+        const elms = document.head.querySelectorAll(
+            'style:not([data-rendered-by-lwc]),link[rel="stylesheet"]'
+        );
+        if (elms.length === lastSeenLength) {
+            return; // nothing to update
+        }
+        lastSeenLength = elms.length;
+        const promises = [...(elms as unknown as Iterable<HTMLStyleElement | HTMLLinkElement>)].map(
+            (elm) => {
+                let promise = elmsToPromises.get(elm);
+                if (!promise) {
+                    // Cache the promise
+                    promise = fetchStylesheet(elm);
+                    elmsToPromises.set(elm, promise);
+                }
+                return promise;
+            }
+        );
+        Promise.all(promises).then((stylesheetTexts) => {
+            // When replaceSync() is called, the entire contents of the constructable stylesheet are replaced
+            // with the copied+concatenated styles. This means that any shadow root's adoptedStyleSheets that
+            // contains this constructable stylesheet will immediately get the new styles.
+            stylesheet.replaceSync(stylesheetTexts.join('\n'));
+        });
+    };
+
+    const headObserver = new MutationObserver(copyToGlobalStylesheet);
+
+    // By observing only the childList, note that we are not covering the case where someone changes an `href`
+    // on an existing <link>`, or the textContent on an existing `<style>`. This is assumed to be an uncommon
+    // case and not worth covering.
+    headObserver.observe(document.head, {
+        childList: true,
+    });
+
+    copyToGlobalStylesheet();
+
+    return stylesheet;
+}
+
+export function applyShadowMigrateMode(shadowRoot: ShadowRoot) {
+    if (!globalStylesheet) {
+        globalStylesheet = initGlobalStylesheet();
+    }
+
+    (shadowRoot as any).synthetic = true; // pretend to be synthetic mode
+    shadowRoot.adoptedStyleSheets.push(globalStylesheet);
+}

packages/@lwc/engine-core/src/framework/vm.ts

@@ -144,6 +144,8 @@ export interface VM<N = HostNode, E = HostElement> {
     /** Rendering operations associated with the VM */
     renderMode: RenderMode;
     shadowMode: ShadowMode;
+    /** True if shadow migrate mode is in effect, i.e. this is native with synthetic-like modifications */
+    shadowMigrateMode: boolean;
     /** The component creation index. */
     idx: number;
     /** Component state, analogous to Element.isConnected */
@@ -353,6 +355,7 @@ export function createVM<HostNode, HostElement>(
         // Properties set right after VM creation.
         tro: null!,
         shadowMode: null!,
+        shadowMigrateMode: false,
         stylesheets: null!,
 
         // Properties set by the LightningElement constructor.
@@ -373,7 +376,13 @@ export function createVM<HostNode, HostElement>(
     }
 
     vm.stylesheets = computeStylesheets(vm, def.ctor);
-    vm.shadowMode = computeShadowMode(def, vm.owner, renderer);
+    const computedShadowMode = computeShadowMode(def, vm.owner, renderer);
+    if (lwcRuntimeFlags.ENABLE_FORCE_SHADOW_MIGRATE_MODE) {
+        vm.shadowMode = ShadowMode.Native;
+        vm.shadowMigrateMode = computedShadowMode === ShadowMode.Synthetic;
+    } else {
+        vm.shadowMode = computedShadowMode;
+    }
     vm.tro = getTemplateReactiveObserver(vm);
 
     // We don't need to report the shadow mode if we're rendering in light DOM
@@ -490,7 +499,8 @@ function computeShadowMode(def: ComponentDef, owner: VM | null, renderer: Render
     const { isSyntheticShadowDefined } = renderer;
 
     let shadowMode;
-    if (isSyntheticShadowDefined) {
+    // If ENABLE_FORCE_SHADOW_MIGRATE_MODE is true, then ShadowMode.Synthetic here will mean "force-migrate" mode.
+    if (isSyntheticShadowDefined || lwcRuntimeFlags.ENABLE_FORCE_SHADOW_MIGRATE_MODE) {
         if (def.renderMode === RenderMode.Light) {
             // ShadowMode.Native implies "not synthetic shadow" which is consistent with how
             // everything defaults to native when the synthetic shadow polyfill is unavailable.

packages/@lwc/engine-dom/src/styles.ts

@@ -62,6 +62,8 @@ function createFreshStyleElement(content: string) {
     const elm = document.createElement('style');
     elm.type = 'text/css';
     elm.textContent = content;
+    // Add an attribute to distinguish global styles added by LWC as opposed to other frameworks/libraries on the page
+    elm.setAttribute('data-rendered-by-lwc', '');
     return elm;
 }
 

packages/@lwc/features/src/index.ts

@@ -18,6 +18,7 @@ const features: FeatureFlagMap = {
     DISABLE_LIGHT_DOM_UNSCOPED_CSS: null,
     ENABLE_FROZEN_TEMPLATE: null,
     ENABLE_LEGACY_SCOPE_TOKENS: null,
+    ENABLE_FORCE_SHADOW_MIGRATE_MODE: null,
 };
 
 // eslint-disable-next-line no-restricted-properties

packages/@lwc/features/src/types.ts

@@ -66,6 +66,10 @@ export interface FeatureFlagMap {
      */
     // TODO [#3733]: remove support for legacy scope tokens
     ENABLE_LEGACY_SCOPE_TOKENS: FeatureFlagValue;
+    /**
+     * If true, enable experimental shadow DOM migration mode globally.
+     */
+    ENABLE_FORCE_SHADOW_MIGRATE_MODE: FeatureFlagValue;
 }
 
 export type FeatureFlagName = keyof FeatureFlagMap;

packages/@lwc/integration-karma/test/native-shadow/force-shadow-migrate-mode/index.spec.js

@@ -0,0 +1,146 @@
+import { createElement, setFeatureFlagForTest } from 'lwc';
+import Native from 'x/native';
+import Synthetic from 'x/synthetic';
+import StyledLight from 'x/styledLight';
+
+function doubleMicrotask() {
+    // wait for applyShadowMigrateMode()
+    return Promise.resolve().then(() => Promise.resolve());
+}
+
+function isActuallyNativeShadow(shadowRoot) {
+    return shadowRoot.constructor.toString().includes('[native code]');
+}
+
+function isClaimingToBeSyntheticShadow(shadowRoot) {
+    return !!shadowRoot.synthetic;
+}
+
+// This test only makes sense for true native shadow mode, because if ENABLE_FORCE_SHADOW_MIGRATE_MODE is true,
+// then the polyfill should not be loaded at all.
+if (process.env.NATIVE_SHADOW && !process.env.MIXED_SHADOW) {
+    describe('shadow migrate mode', () => {
+        beforeEach(async () => {
+            const style = document.createElement('style');
+            style.textContent = 'h1 { color: blue }';
+            document.head.appendChild(style);
+        });
+
+        describe('flag on', () => {
+            beforeEach(() => {
+                setFeatureFlagForTest('ENABLE_FORCE_SHADOW_MIGRATE_MODE', true);
+            });
+
+            afterEach(() => {
+                setFeatureFlagForTest('ENABLE_FORCE_SHADOW_MIGRATE_MODE', false);
+            });
+
+            it('uses global styles for synthetic components', async () => {
+                const elm = createElement('x-synthetic', { is: Synthetic });
+                document.body.appendChild(elm);
+
+                await doubleMicrotask();
+
+                expect(isActuallyNativeShadow(elm.shadowRoot)).toBe(true);
+                expect(isClaimingToBeSyntheticShadow(elm.shadowRoot)).toBe(true);
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).color).toBe(
+                    'rgb(0, 0, 255)'
+                );
+            });
+
+            it('does not use global styles for native components', async () => {
+                const elm = createElement('x-native', { is: Native });
+                document.body.appendChild(elm);
+
+                await doubleMicrotask();
+
+                expect(isActuallyNativeShadow(elm.shadowRoot)).toBe(true);
+                expect(isClaimingToBeSyntheticShadow(elm.shadowRoot)).toBe(false);
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).color).toBe(
+                    'rgb(0, 0, 0)'
+                );
+            });
+
+            it('does not apply styles from global light DOM components to synthetic components', async () => {
+                const light = createElement('x-styled-light', { is: StyledLight });
+                document.body.appendChild(light);
+
+                const elm = createElement('x-synthetic', { is: Synthetic });
+                document.body.appendChild(elm);
+
+                await doubleMicrotask();
+
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).opacity).toBe('1');
+                expect(getComputedStyle(light.querySelector('h1')).opacity).toBe('0.5');
+            });
+
+            it('uses new styles added to the head after component is rendered', async () => {
+                const elm = createElement('x-synthetic', { is: Synthetic });
+                document.body.appendChild(elm);
+
+                await doubleMicrotask();
+
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).color).toBe(
+                    'rgb(0, 0, 255)'
+                );
+
+                const style = document.createElement('style');
+                style.textContent = `h1 { color: purple; background-color: crimson }`;
+                document.head.appendChild(style);
+
+                await doubleMicrotask();
+
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).color).toBe(
+                    'rgb(128, 0, 128)'
+                );
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).backgroundColor).toBe(
+                    'rgb(220, 20, 60)'
+                );
+            });
+
+            it('local styles are defined after global styles', async () => {
+                const elm = createElement('x-synthetic', { is: Synthetic });
+                document.body.appendChild(elm);
+
+                const style = document.createElement('style');
+                style.textContent = `h1 { font-family: sans-serif; background-color: green; }`;
+                document.head.appendChild(style);
+
+                await doubleMicrotask();
+
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).backgroundColor).toBe(
+                    'rgb(0, 128, 0)'
+                );
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).fontFamily).toBe(
+                    'monospace'
+                );
+            });
+        });
+
+        describe('flag off', () => {
+            it('does not use global styles for synthetic components', async () => {
+                const elm = createElement('x-synthetic', { is: Synthetic });
+                document.body.appendChild(elm);
+
+                await doubleMicrotask();
+                expect(isActuallyNativeShadow(elm.shadowRoot)).toBe(true);
+                expect(isClaimingToBeSyntheticShadow(elm.shadowRoot)).toBe(false);
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).color).toBe(
+                    'rgb(0, 0, 0)'
+                );
+            });
+
+            it('does not use global styles for native components', async () => {
+                const elm = createElement('x-native', { is: Native });
+                document.body.appendChild(elm);
+
+                await doubleMicrotask();
+                expect(isActuallyNativeShadow(elm.shadowRoot)).toBe(true);
+                expect(isClaimingToBeSyntheticShadow(elm.shadowRoot)).toBe(false);
+                expect(getComputedStyle(elm.shadowRoot.querySelector('h1')).color).toBe(
+                    'rgb(0, 0, 0)'
+                );
+            });
+        });
+    });
+}

packages/@lwc/integration-karma/test/native-shadow/force-shadow-migrate-mode/x/native/native.html

@@ -0,0 +1,3 @@
+<template>
+    <h1>Hello</h1>
+</template>

packages/@lwc/integration-karma/test/native-shadow/force-shadow-migrate-mode/x/native/native.js

@@ -0,0 +1,5 @@
+import { LightningElement } from 'lwc';
+
+export default class extends LightningElement {
+    static shadowSupportMode = 'native';
+}

packages/@lwc/integration-karma/test/native-shadow/force-shadow-migrate-mode/x/styledLight/styledLight.css

@@ -0,0 +1,3 @@
+h1 {
+    opacity: 0.5;
+}

packages/@lwc/integration-karma/test/native-shadow/force-shadow-migrate-mode/x/styledLight/styledLight.html

@@ -0,0 +1,3 @@
+<template lwc:render-mode="light">
+    <h1>Hello</h1>
+</template>

packages/@lwc/integration-karma/test/native-shadow/force-shadow-migrate-mode/x/styledLight/styledLight.js

@@ -0,0 +1,5 @@
+import { LightningElement } from 'lwc';
+
+export default class extends LightningElement {
+    static renderMode = 'light';
+}

packages/@lwc/integration-karma/test/native-shadow/force-shadow-migrate-mode/x/synthetic/synthetic.css

@@ -0,0 +1,3 @@
+h1 {
+    font-family: monospace;
+}

packages/@lwc/integration-karma/test/native-shadow/force-shadow-migrate-mode/x/synthetic/synthetic.html

@@ -0,0 +1,3 @@
+<template>
+    <h1>Hello</h1>
+</template>

packages/@lwc/integration-karma/test/native-shadow/force-shadow-migrate-mode/x/synthetic/synthetic.js

@@ -0,0 +1,3 @@
+import { LightningElement } from 'lwc';
+
+export default class extends LightningElement {}