Scroll the position into view when the content is not visible.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights
  * reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
@@ skipping 1627 matching lines @@
   LayoutRect layerBounds(LayoutPoint(),
                          LayoutSize(box().clientWidth(), box().clientHeight()));
   LayoutRect r = ScrollAlignment::getRectToExpose(layerBounds, localExposeRect,
                                                   alignX, alignY);
 
   ScrollOffset oldScrollOffset = getScrollOffset();
   ScrollOffset newScrollOffset(clampScrollOffset(roundedIntSize(
       toScrollOffset(FloatPoint(r.location()) + oldScrollOffset))));
 
   if (newScrollOffset == oldScrollOffset) {
-    return LayoutRect(
-        box()
-            .localToAbsoluteQuad(FloatQuad(FloatRect(intersection(
-                                     layerBounds, localExposeRect))),
-                                 UseTransforms)
-            .boundingBox());
+    LayoutRect intersect =
+        LayoutRect(box()
+                       .localToAbsoluteQuad(FloatQuad(FloatRect(intersection(

[bokan, 2016/12/15 21:31:19]

this long call chain is really obscuring the intent here. Could you put this
into a static helper above, something like:

LayoutRect localToAbsolute(LayoutBox, LayoutRect)

[sunyunjia, 2016/12/19 18:05:44]

Done.

+                                                layerBounds, localExposeRect))),
+                                            UseTransforms)
+                       .boundingBox());
+    if (intersect.isEmpty() && !layerBounds.isEmpty() &&
+        !localExposeRect.isEmpty()) {
+      return LayoutRect(
+          box()
+              .localToAbsoluteQuad(FloatQuad(FloatRect(layerBounds)),
+                                   UseTransforms)
+              .boundingBox());
+    }
+    return intersect;
   }
 
   setScrollOffset(newScrollOffset, scrollType, ScrollBehaviorInstant);
   ScrollOffset scrollOffsetDifference = getScrollOffset() - oldScrollOffset;
   localExposeRect.move(-LayoutSize(scrollOffsetDifference));
-  return LayoutRect(
+  LayoutRect intersect = LayoutRect(
       box()
           .localToAbsoluteQuad(
               FloatQuad(FloatRect(intersection(layerBounds, localExposeRect))),
               UseTransforms)
           .boundingBox());
+  if (intersect.isEmpty() && !layerBounds.isEmpty() &&

[bokan, 2016/12/15 21:31:19]

Hmm, doesn't this break the case that we introduced the intersection logic for?
Namely crbug.com/437025

[sunyunjia, 2016/12/19 18:05:44]

I have to say that the bug itself is an edge case. It is trying to scroll an
invisible element into view (the element can not be scrolled into its closest
scrollable area). In this case, the intersection of the layerbounds and
localExposeRect is empty. We assume the page just want to scroll the position of
the element into view, and doesn't care whether the element is visible.

+      !localExposeRect.isEmpty()) {
+    return LayoutRect(box()
+                          .localToAbsoluteQuad(
+                              FloatQuad(FloatRect(layerBounds)), UseTransforms)
+                          .boundingBox());
+  }
+  return intersect;
 }
 
 void PaintLayerScrollableArea::updateScrollableAreaSet(bool hasOverflow) {
   LocalFrame* frame = box().frame();
   if (!frame)
     return;
 
   FrameView* frameView = frame->view();
   if (!frameView)
     return;
@@ skipping 359 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetAfterOverflowChange();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink