Determine the layoutobject of middleClickAutoscroll by the scroll direction.

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc.
  *               All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 1065 matching lines @@
     previousMousePosition = lastKnownMousePosition;
 
   IntSize delta = lastKnownMousePosition - sourcePoint;
 
   // at the center we let the space for the icon.
   if (abs(delta.width()) <= AutoscrollController::noMiddleClickAutoscrollRadius)
     delta.setWidth(0);
   if (abs(delta.height()) <=
       AutoscrollController::noMiddleClickAutoscrollRadius)
     delta.setHeight(0);
-  scroll(ScrollByPixel, FloatSize(adjustedScrollDelta(delta)));
+  ScrollResult result =
+      scroll(ScrollByPixel, FloatSize(adjustedScrollDelta(delta)));
+  AutoscrollController& autoscrollController =
+      frame->page()->autoscrollController();
+
+  if (!delta.isZero() && !result.didScroll()) {

[bokan, 2016/11/09 15:34:10]

We should put all this logic inside AutoscrollController

+    LayoutObject* layoutObject = this;
+    if (node() && node()->isDocumentNode()) {
+      Element* owner = toDocument(layoutObject->node())->localOwner();
+      layoutObject = owner ? owner->layoutObject() : nullptr;
+    } else {
+      layoutObject = parent();
+    }
+    while (layoutObject &&
+           (!layoutObject->isBox() ||
+            !toLayoutBox(layoutObject)->canBeScrolledAndHasScrollableArea())) {
+      if (layoutObject->node() && layoutObject->node()->isDocumentNode()) {

[bokan, 2016/11/09 15:34:10]

This will just pick the next scroller up in the hierarchy but that doesn't
necessarily mean it's scrollable in the direction. E.g. if we want to scroll
vertically but the vertical scroller has two nested horizontal scrollers. I
think you need to call scroll() from this loop.

+        Element* owner = toDocument(layoutObject->node())->localOwner();
+        layoutObject = owner ? owner->layoutObject() : nullptr;
+      } else {
+        layoutObject = layoutObject->parent();
+      }
+    }
+    if (autoscrollController.setMiddleClickAutoscrollLayoutObject(
+            toLayoutBox(layoutObject))) {
+      toLayoutBox(layoutObject)->middleClickAutoscroll(sourcePoint);
+    }
+  } else if (result.didScroll()) {
+    autoscrollController.setMiddleClickAutoscrolled(true);
+  }
 }
 
 void LayoutBox::scrollByRecursively(const ScrollOffset& delta) {
   if (delta.isZero())
     return;
 
   bool restrictedByLineClamp = false;
   if (parent())
     restrictedByLineClamp = !parent()->style()->lineClamp().isNone();
 
@@ skipping 4541 matching lines @@
   LayoutRect rect = frameRect();
 
   LayoutBlock* block = containingBlock();
   if (block)
     block->adjustChildDebugRect(rect);
 
   return rect;
 }
 
 }  // namespace blink