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 68 matching lines @@
 #include "platform/PlatformMouseEvent.h"
 #include "platform/graphics/CompositorMutableProperties.h"
 #include "platform/graphics/GraphicsLayer.h"
 #include "platform/graphics/paint/DrawingRecorder.h"
 #include "platform/scroll/ScrollAnimatorBase.h"
 #include "platform/scroll/ScrollbarTheme.h"
 #include "public/platform/Platform.h"
 
 namespace blink {
 
+static LayoutRect localToAbsolute(LayoutBox& offset, LayoutRect rect) {
+  return LayoutRect(
+      offset.localToAbsoluteQuad(FloatQuad(FloatRect(rect)), UseTransforms)
+          .boundingBox());
+}
+
 PaintLayerScrollableAreaRareData::PaintLayerScrollableAreaRareData() {}
 
 const int ResizerControlExpandRatioForTouch = 2;
 
 PaintLayerScrollableArea::PaintLayerScrollableArea(PaintLayer& layer)
     : m_layer(layer),
       m_nextTopmostScrollChild(0),
       m_topmostScrollChild(0),
       m_inResizeMode(false),
       m_scrollsOverflow(false),
@@ skipping 1538 matching lines @@
   localExposeRect.move(-box().borderLeft(), -box().borderTop());
   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());
+  if (newScrollOffset != oldScrollOffset) {

[bokan, 2017/01/04 19:41:24]

I think there's no reason to keep the `if`, it's still correct if
newScrollOffset == oldScrollOffset, it'll just move by 0

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