fix(slide-toggle): drag not working in edge

Some browsers like Edge or IE11 emit `click` events differently than other browsers like Chrome. For example does Edge still emit a `click` event if the pointer drags something around and is being released over the `<label>` element. This doesn't happen on Chrome.

Right now this means that the new checked state from the drag is being reverted by the `click` event that then causes a `change` event on the underlying input element. To ensure that the behavior is the same across all supported browsers, the checked state from the `click` event is being ignored while dragging.

Fixes angular#8391

src/lib/slide-toggle/slide-toggle.spec.ts

@@ -475,6 +475,29 @@ describe('MatSlideToggle without forms', () => {
           .toBeFalsy('Expected the slide-toggle to not emit a change event.');
     }));
 
+    it('should ignore clicks on the label element while dragging', fakeAsync(() => {
+      expect(slideToggle.checked).toBe(false);
+
+      gestureConfig.emitEventForElement('slidestart', slideThumbContainer);
+      gestureConfig.emitEventForElement('slide', slideThumbContainer, {
+        deltaX: 200 // Arbitrary, large delta that will be clamped to the end of the slide-toggle.
+      });
+      gestureConfig.emitEventForElement('slideend', slideThumbContainer);
+
+      expect(slideToggle.checked).toBe(true);
+
+      // Fake a change event that has been fired after dragging through the click on pointer
+      // release (noticeable on IE11, Edge)
+      inputElement.checked = false;
+      dispatchFakeEvent(inputElement, 'change');
+
+      // Flush the timeout for the slide ending.
+      tick();
+
+      expect(slideThumbContainer.classList).not.toContain('mat-dragging');
+      expect(slideToggle.checked).toBe(true);
+    }));
+
     it('should update the checked property of the input', fakeAsync(() => {
       expect(inputElement.checked).toBe(false);
 

src/lib/slide-toggle/slide-toggle.ts

@@ -161,32 +161,31 @@ export class MatSlideToggle extends _MatSlideToggleMixinBase implements OnDestro
     this._focusMonitor.stopMonitoring(this._inputElement.nativeElement);
   }
 
-  /**
-   * This function will called if the underlying input changed its value through user interaction.
-   */
+  /** Method being called whenever the underlying input emits a change event. */
   _onChangeEvent(event: Event) {
     // We always have to stop propagation on the change event.
     // Otherwise the change event, from the input element, will bubble up and
     // emit its event object to the component's `change` output.
     event.stopPropagation();
 
-    // Sync the value from the underlying input element with the slide-toggle component.
+    // Releasing the pointer over the `<label>` element while dragging triggers another
+    // click event on the `<label>` element. This means that the checked state of the underlying
+    // input changed unintentionally and needs to be changed back.
+    if (this._slideRenderer.dragging) {
+      this._inputElement.nativeElement.checked = this.checked;
+      return;
+    }
+
+    // Sync the value from the underlying input element with the component instance.
     this.checked = this._inputElement.nativeElement.checked;
 
-    // Emit our custom change event if the native input emitted one.
-    // It is important to only emit it, if the native input triggered one, because we don't want
-    // to trigger a change event, when the `checked` variable changes programmatically.
+    // Emit our custom change event only if the underlying input emitted one. This ensures that
+    // there is no change event, when the checked state changes programmatically.
     this._emitChangeEvent();
   }
 
+  /** Method being called whenever the slide-toggle has been clicked. */
   _onInputClick(event: Event) {
-    // In some situations the user will release the mouse on the label element. The label element
-    // redirects the click to the underlying input element and will result in a value change.
-    // Prevent the default behavior if dragging, because the value will be set after drag.
-    if (this._slideRenderer.dragging) {
-      event.preventDefault();
-    }
-
     // We have to stop propagation for click events on the visual hidden input element.
     // By default, when a user clicks on a label element, a generated click event will be
     // dispatched on the associated input element. Since we are using a label element as our
@@ -269,10 +268,10 @@ export class MatSlideToggle extends _MatSlideToggleMixinBase implements OnDestro
 
   _onDragEnd() {
     if (this._slideRenderer.dragging) {
-      let _previousChecked = this.checked;
-      this.checked = this._slideRenderer.dragPercentage > 50;
+      const newCheckedValue = this._slideRenderer.dragPercentage > 50;
 
-      if (_previousChecked !== this.checked) {
+      if (newCheckedValue !== this.checked) {
+        this.checked = newCheckedValue;
         this._emitChangeEvent();
       }