From 653dda32baa494968308691e98f41d2dd16df746 Mon Sep 17 00:00:00 2001
From: Kevin Kimball <kwkimball@google.com>
Date: Mon, 3 Feb 2020 15:49:23 -0800
Subject: [PATCH] =?UTF-8?q?=F0=9F=90=9BForce=20scrollTop=20to=20be=20remea?=
 =?UTF-8?q?sured=20on=20shadow=20doc=20visibility=20change=20(#26588)?=
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* Force scrolltop to be remeasured on shadow doc visibility change

* Add comment to fix
---
 src/service/viewport/viewport-impl.js |  5 +++++
 test/unit/test-viewport.js            | 21 +++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/src/service/viewport/viewport-impl.js b/src/service/viewport/viewport-impl.js
index d1e5fd0a49f0..b2a1f20e3493 100644
--- a/src/service/viewport/viewport-impl.js
+++ b/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();
       }
diff --git a/test/unit/test-viewport.js b/test/unit/test-viewport.js
index 5f4dc418118b..da56cdb72a11 100644
--- a/test/unit/test-viewport.js
+++ b/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');