🐛Force scrollTop to be remeasured on shadow doc visibility change (#2…

…6588)

* Force scrolltop to be remeasured on shadow doc visibility change

* Add comment to fix

src/service/viewport/viewport-impl.js

@@ -228,6 +228,11 @@ export class ViewportImpl {
           // the size has changed between `disconnect` and `connect`.
           this.resize_();
         }
+        if (this.scrollTop_) {
+          // Remeasure scrollTop when resource becomes visible to fix #11983
+          this./*OK*/ scrollTop_ = null;
+          this.getScrollTop();
+        }
       } else {
         this.binding_.disconnect();
       }

test/unit/test-viewport.js

@@ -395,6 +395,27 @@ describes.fakeWin('Viewport', {}, env => {
     expect(binding.disconnect).to.be.calledOnce;
   });
 
+  it('should update scroll position when visibility changes', () => {
+    binding = new ViewportBindingDef();
+    binding.getScrollTop = (() => {
+      const generator = (function*() {
+        yield 25;
+        return 100;
+      })();
+      return () => generator.next().value;
+    })();
+    viewport = new ViewportImpl(ampdoc, binding, viewer);
+
+    // Force scrollTop to be measured
+    viewport.getScrollTop();
+    expect(viewport./*OK*/ scrollTop_).to.equal(25);
+    // Toggle visibility state
+    changeVisibilityState('prerender');
+    changeVisibilityState('visible');
+    // Expect scrollTop to be remeasured
+    expect(viewport./*OK*/ scrollTop_).to.equal(100);
+  });
+
   it('should resize only after size has been initialed', () => {
     onVisibilityHandlers.length = 0;
     changeVisibilityState('visible');