fix(page): teach page.click() to click partially offscreen buttons (#2806)

Originally, we use `Element.scrollIntoViewIfNeeded` to make sure
button is on screen before trying to click it.

However, `Element.scrollIntoViewIfNeeded` doesn't work in certain
scenarios, e.g. when element is partially visible and horizontal
scrolling is required to make it fully visible.

This patch polyfills `element.scrollIntoViewIfNeeded` using
IntersectionObserver and `Element.scrollIntoView`.

Fixes #2804.

lib/ElementHandle.js

@@ -55,12 +55,20 @@ class ElementHandle extends JSHandle {
   }
 
   async _scrollIntoViewIfNeeded() {
-    const error = await this.executionContext().evaluate(element => {
+    const error = await this.executionContext().evaluate(async element => {
       if (!element.isConnected)
         return 'Node is detached from document';
       if (element.nodeType !== Node.ELEMENT_NODE)
         return 'Node is not of type HTMLElement';
-      element.scrollIntoViewIfNeeded();
+      const visibleRatio = await new Promise(resolve => {
+        const observer = new IntersectionObserver(entries => {
+          resolve(entries[0].intersectionRatio);
+          observer.disconnect();
+        });
+        observer.observe(element);
+      });
+      if (visibleRatio !== 1.0)
+        element.scrollIntoView({block: 'center', inline: 'center', behavior: 'instant'});
       return false;
     }, this);
     if (error)
@@ -70,8 +78,7 @@ class ElementHandle extends JSHandle {
   /**
    * @return {!Promise<{x: number, y: number}>}
    */
-  async _visibleCenter() {
+  async _boundingBoxCenter() {
-    await this._scrollIntoViewIfNeeded();
     const box = await this._assertBoundingBox();
     return {
       x: box.x + box.width / 2,
@@ -102,7 +109,8 @@ class ElementHandle extends JSHandle {
   }
 
   async hover() {
-    const {x, y} = await this._visibleCenter();
+    await this._scrollIntoViewIfNeeded();
+    const {x, y} = await this._boundingBoxCenter();
     await this._page.mouse.move(x, y);
   }
 
@@ -110,7 +118,8 @@ class ElementHandle extends JSHandle {
    * @param {!Object=} options
    */
   async click(options = {}) {
-    const {x, y} = await this._visibleCenter();
+    await this._scrollIntoViewIfNeeded();
+    const {x, y} = await this._boundingBoxCenter();
     await this._page.mouse.click(x, y, options);
   }
 
@@ -125,7 +134,8 @@ class ElementHandle extends JSHandle {
   }
 
   async tap() {
-    const {x, y} = await this._visibleCenter();
+    await this._scrollIntoViewIfNeeded();
+    const {x, y} = await this._boundingBoxCenter();
     await this._page.touchscreen.tap(x, y);
   }
 
@@ -222,9 +232,7 @@ class ElementHandle extends JSHandle {
       needsViewportReset = true;
     }
 
-    await this.executionContext().evaluate(function(element) {
+    await this._scrollIntoViewIfNeeded();
-      element.scrollIntoView({block: 'center', inline: 'center', behavior: 'instant'});
-    }, this);
 
     boundingBox = await this._assertBoundingBox();
 

test/assets/offscreenbuttons.html

@@ -0,0 +1,34 @@
+<style>
+  button {
+    position: absolute;
+    width: 100px;
+    height: 20px;
+  }
+
+  #btn0 { right: 0px; top: 0; }
+  #btn1 { right: -10px; top: 25px; }
+  #btn2 { right: -20px; top: 50px; }
+  #btn3 { right: -30px; top: 75px; }
+  #btn4 { right: -40px; top: 100px; }
+  #btn5 { right: -50px; top: 125px; }
+  #btn6 { right: -60px; top: 150px; }
+  #btn7 { right: -70px; top: 175px; }
+  #btn8 { right: -80px; top: 200px; }
+  #btn9 { right: -90px; top: 225px; }
+</style>
+<button id=btn0>0</button>
+<button id=btn1>1</button>
+<button id=btn2>2</button>
+<button id=btn3>3</button>
+<button id=btn4>4</button>
+<button id=btn5>5</button>
+<button id=btn6>6</button>
+<button id=btn7>7</button>
+<button id=btn8>8</button>
+<button id=btn9>9</button>
+<script>
+window.addEventListener('DOMContentLoaded', () => {
+  for (const button of Array.from(document.querySelectorAll('button')))
+    button.addEventListener('click', () => console.log('button #' + button.textContent + ' clicked'), false);
+}, false);
+</script>

test/input.spec.js

@@ -29,6 +29,29 @@ module.exports.addTests = function({testRunner, expect, DeviceDescriptors}) {
       expect(await page.evaluate(() => result)).toBe('Clicked');
     });
 
+    it('should click offscreen buttons', async({page, server}) => {
+      await page.goto(server.PREFIX + '/offscreenbuttons.html');
+      const messages = [];
+      page.on('console', msg => messages.push(msg.text()));
+      for (let i = 0; i < 10; ++i) {
+        // We might've scrolled to click a button - reset to (0, 0).
+        await page.evaluate(() => window.scrollTo(0, 0));
+        await page.click(`#btn${i}`);
+      }
+      expect(messages).toEqual([
+        'button #0 clicked',
+        'button #1 clicked',
+        'button #2 clicked',
+        'button #3 clicked',
+        'button #4 clicked',
+        'button #5 clicked',
+        'button #6 clicked',
+        'button #7 clicked',
+        'button #8 clicked',
+        'button #9 clicked'
+      ]);
+    });
+
     it('should click on checkbox input and toggle', async({page, server}) => {
       await page.goto(server.PREFIX + '/input/checkbox.html');
       expect(await page.evaluate(() => result.check)).toBe(null);