🐛 amp-base-carousel: Update offset and index together

extensions/amp-base-carousel/0.1/carousel.js

@@ -1180,10 +1180,11 @@ export class Carousel {
     }
 
     // We are updating during a programmatic scroll, so go to the correct
-    // index.
+    // index (and update offset accordingly).
     if (this.requestedIndex_ !== null) {
       this.currentIndex_ = this.requestedIndex_;
       this.requestedIndex_ = null;
+      this.currentElementOffset_ = 0;
     }
 
     const totalLength = sum(this.getSlideLengths_());

extensions/amp-base-carousel/0.1/test/test-carousel.js

@@ -236,4 +236,30 @@ describes.realWin('carousel implementation', {}, (env) => {
       expect(carousel.isAtStart()).to.be.false;
     });
   });
+
+  describe('resetScrollReferencePoint_', () => {
+    it('currentElementOffset_ & currentIndex_ should be set when it is a' + 
+    ' programmatic scroll', async () => {
+      setStyle(element, 'width', '299.2px');
+
+      const carousel = await createCarousel({
+        slideCount: 12,
+        loop: false,
+      });
+
+      // Fake the scroll that ends short of the correct index.
+      // This is handled by scroll event listener.
+      carousel.touching_ = false;
+      carousel.requestedIndex_ = 1;
+      carousel.currentIndex_ = 0;
+      carousel.restingIndex_ = 0;
+      carousel.currentElementOffset_ = -0.99382;
+
+      carousel.resetScrollReferencePoint_();
+
+      expect(carousel.currentElementOffset_).to.equal(0);
+      expect(carousel.currentIndex_).to.equal(1);
+      expect(carousel.requestedIndex_).to.be.null;
+    });
+  });
 });