fix(slider): stop dragging if page loses focus (#17849)

If the page loses focus (e.g. from an `alert` or the window being minimized) while the user is dragging a slider, it'll be stuck as dragging until the user comes back and clicks somewhere which might look weird and change the value. These changes stop the dragging sequence once the page is blurred.

src/material/slider/slider.spec.ts

@@ -139,6 +139,27 @@ describe('MatSlider', () => {
       expect(sliderNativeElement.classList).not.toContain('mat-slider-sliding');
     });
 
+    it('should stop dragging if the page loses focus', () => {
+      const classlist = sliderNativeElement.classList;
+
+      expect(classlist).not.toContain('mat-slider-sliding');
+
+      dispatchSlideStartEvent(sliderNativeElement, 0);
+      fixture.detectChanges();
+
+      expect(classlist).toContain('mat-slider-sliding');
+
+      dispatchSlideEvent(sliderNativeElement, 0.34);
+      fixture.detectChanges();
+
+      expect(classlist).toContain('mat-slider-sliding');
+
+      dispatchFakeEvent(window, 'blur');
+      fixture.detectChanges();
+
+      expect(classlist).not.toContain('mat-slider-sliding');
+    });
+
     it('should reset active state upon blur', () => {
       sliderInstance._isActive = true;
 

src/material/slider/slider.ts

@@ -475,6 +475,9 @@ export class MatSlider extends _MatSliderMixinBase
     return (this._dir && this._dir.value == 'rtl') ? 'rtl' : 'ltr';
   }
 
+  /** Keeps track of the last pointer event that was captured by the slider. */
+  private _lastPointerEvent: MouseEvent | TouchEvent | null;
+
   constructor(elementRef: ElementRef,
               private _focusMonitor: FocusMonitor,
               private _changeDetectorRef: ChangeDetectorRef,
@@ -513,6 +516,7 @@ export class MatSlider extends _MatSliderMixinBase
     const element = this._elementRef.nativeElement;
     element.removeEventListener('mousedown', this._pointerDown, activeEventOptions);
     element.removeEventListener('touchstart', this._pointerDown, activeEventOptions);
+    this._lastPointerEvent = null;
     this._removeGlobalEvents();
     this._focusMonitor.stopMonitoring(this._elementRef);
     this._dirChangeSubscription.unsubscribe();
@@ -611,6 +615,7 @@ export class MatSlider extends _MatSliderMixinBase
       const oldValue = this.value;
       const pointerPosition = getPointerPositionOnPage(event);
       this._isSliding = true;
+      this._lastPointerEvent = event;
       event.preventDefault();
       this._focusHostElement();
       this._onMouseenter(); // Simulate mouseenter in case this is a mobile device.
@@ -637,6 +642,7 @@ export class MatSlider extends _MatSliderMixinBase
       // Prevent the slide from selecting anything else.
       event.preventDefault();
       const oldValue = this.value;
+      this._lastPointerEvent = event;
       this._updateValueFromPosition(getPointerPositionOnPage(event));
 
       // Native range elements always emit `input` events when the value changed while sliding.
@@ -654,7 +660,7 @@ export class MatSlider extends _MatSliderMixinBase
 
       event.preventDefault();
       this._removeGlobalEvents();
-      this._valueOnSlideStart = this._pointerPositionOnStart = null;
+      this._valueOnSlideStart = this._pointerPositionOnStart = this._lastPointerEvent = null;
       this._isSliding = false;
 
       if (this._valueOnSlideStart != this.value && !this.disabled &&
@@ -665,6 +671,15 @@ export class MatSlider extends _MatSliderMixinBase
     }
   }
 
+  /** Called when the window has lost focus. */
+  private _windowBlur = () => {
+    // If the window is blurred while dragging we need to stop dragging because the
+    // browser won't dispatch the `mouseup` and `touchend` events anymore.
+    if (this._lastPointerEvent) {
+      this._pointerUp(this._lastPointerEvent);
+    }
+  }
+
   /**
    * Binds our global move and end events. They're bound at the document level and only while
    * dragging so that the user doesn't have to keep their pointer exactly over the slider
@@ -683,6 +698,9 @@ export class MatSlider extends _MatSliderMixinBase
         body.addEventListener('touchcancel', this._pointerUp, activeEventOptions);
       }
     }
+    if (typeof window !== 'undefined' && window) {
+      window.addEventListener('blur', this._windowBlur);
+    }
   }
 
   /** Removes any global event listeners that we may have added. */
@@ -695,6 +713,9 @@ export class MatSlider extends _MatSliderMixinBase
       body.removeEventListener('touchend', this._pointerUp, activeEventOptions);
       body.removeEventListener('touchcancel', this._pointerUp, activeEventOptions);
     }
+    if (typeof window !== 'undefined' && window) {
+      window.removeEventListener('blur', this._windowBlur);
+    }
   }
 
   /** Increments the slider by the given number of steps (negative number decrements). */