fix(waitForSelector): use raf polling instead of mutation (#2047)

MutationObserver does not work with mutations in the shadow, so we cannot use it for selectors that pierce shadows.

src/selectors.ts

@@ -145,8 +145,7 @@ export class Selectors {
   _waitForSelectorTask(selector: string, waitFor: 'attached' | 'detached' | 'visible' | 'hidden', deadline: number): { world: 'main' | 'utility', task: (context: dom.FrameExecutionContext) => Promise<js.JSHandle> } {
     const parsed = this._parseSelector(selector);
     const task = async (context: dom.FrameExecutionContext) => context.evaluateHandleInternal(({ evaluator, parsed, waitFor, timeout }) => {
-      const polling = (waitFor === 'attached' || waitFor === 'detached') ? 'mutation' : 'raf';
+      return evaluator.injected.poll('raf', timeout, () => {
-      return evaluator.injected.poll(polling, timeout, () => {
         const element = evaluator.querySelector(parsed, document);
         switch (waitFor) {
           case 'attached':
@@ -166,7 +165,7 @@ export class Selectors {
   _dispatchEventTask(selector: string, type: string, eventInit: Object, deadline: number): (context: dom.FrameExecutionContext) => Promise<js.JSHandle> {
     const parsed = this._parseSelector(selector);
     const task = async (context: dom.FrameExecutionContext) => context.evaluateHandleInternal(({ evaluator, parsed, type, eventInit, timeout }) => {
-      return evaluator.injected.poll('mutation', timeout, () => {
+      return evaluator.injected.poll('raf', timeout, () => {
         const element = evaluator.querySelector(parsed, document);
         if (element)
           evaluator.injected.dispatchEvent(element, type, eventInit);

test/dispatchevent.spec.js

@@ -79,6 +79,24 @@ describe('Page.dispatchEvent(click)', function() {
     await page.dispatchEvent('button', 'click');
     expect(await page.evaluate(() => window.clicked)).toBeTruthy();
   });
+  it('should dispatch click when node is added in shadow dom', async({page, server}) => {
+    await page.goto(server.EMPTY_PAGE);
+    const watchdog = page.dispatchEvent('span', 'click');
+    await page.evaluate(() => {
+      const div = document.createElement('div');
+      div.attachShadow({mode: 'open'});
+      document.body.appendChild(div);
+    });
+    await page.evaluate(() => new Promise(f => setTimeout(f, 100)));
+    await page.evaluate(() => {
+      const span = document.createElement('span');
+      span.textContent = 'Hello from shadow';
+      span.addEventListener('click', () => window.clicked = true);
+      document.querySelector('div').shadowRoot.appendChild(span);
+    });
+    await watchdog;
+    expect(await page.evaluate(() => window.clicked)).toBe(true);
+  });
 });
 
 describe('Page.dispatchEvent(drag)', function() {

test/waittask.spec.js

@@ -192,6 +192,23 @@ describe('Frame.waitForSelector', function() {
     const tagName = await eHandle.getProperty('tagName').then(e => e.jsonValue());
     expect(tagName).toBe('DIV');
   });
+  it('should resolve promise when node is added in shadow dom', async({page, server}) => {
+    await page.goto(server.EMPTY_PAGE);
+    const watchdog = page.waitForSelector('span');
+    await page.evaluate(() => {
+      const div = document.createElement('div');
+      div.attachShadow({mode: 'open'});
+      document.body.appendChild(div);
+    });
+    await page.evaluate(() => new Promise(f => setTimeout(f, 100)));
+    await page.evaluate(() => {
+      const span = document.createElement('span');
+      span.textContent = 'Hello from shadow';
+      document.querySelector('div').shadowRoot.appendChild(span);
+    });
+    const handle = await watchdog;
+    expect(await handle.evaluate(e => e.textContent)).toBe('Hello from shadow');
+  });
   it('should work when node is added through innerHTML', async({page, server}) => {
     await page.goto(server.EMPTY_PAGE);
     const watchdog = page.waitForSelector('h3 div');