Limit ScrollAnchor to 20 adjustments between user scrolls.

third_party/WebKit/Source/core/layout/ScrollAnchor.cpp

Index: third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
diff --git a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
index 8d1de54353f6f1b6df13ce4e08afc9fb9725ba54..48b648de6111c3ad0c8eb8f2742b2dce9cf49b3d 100644
--- a/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
+++ b/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
@@ -14,9 +14,12 @@ namespace blink {
 
 using Corner = ScrollAnchor::Corner;
 
+static const int kMaxAdjustments = 20;

[ymalik, 2016/07/01 01:07:11]

How did you come up with this number? We do have some UMA metrics that may be
helpful in picking this. 

If I understand correctly, with this change, in the techcunch header bounce
case, we will now see it stop after bouncing for a little bit?

[skobes, 2016/07/01 17:28:26]

I tried to pick something high enough to preserve adjustments when an ad
animates its expansion, but low enough that you don't have to wait a long time
to break out of the infinite loop case.  The video I posted to the bug shows
what it looks like.

+
 ScrollAnchor::ScrollAnchor(ScrollableArea* scroller)
     : m_scroller(scroller)
     , m_hasBounced(false)
+    , m_adjustmentCount(0)
 {
     ASSERT(m_scroller);
     ASSERT(m_scroller->isFrameView() || m_scroller->isPaintLayerScrollableArea());
@@ -251,7 +254,8 @@ void ScrollAnchor::restore()
         m_lastAdjusted.clear();
         return;
     }
-    adjust(adjustment);
+    if (++m_adjustmentCount <= kMaxAdjustments)

[ymalik, 2016/07/01 01:07:11]

Brief comment explaining the rationale behind this?

[skobes, 2016/07/01 17:28:26]

Done.

+        adjust(adjustment);
 }
 
 void ScrollAnchor::adjust(IntSize adjustment)
@@ -274,6 +278,7 @@ void ScrollAnchor::adjust(IntSize adjustment)
 
 void ScrollAnchor::clear()
 {
+    m_adjustmentCount = 0;
     m_current.clear();
 }