Determine the layoutobject of middleClickAutoscroll by the scroll direction.

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

Index: third_party/WebKit/Source/core/layout/LayoutBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBox.cpp b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
index eadfef9c82765fe03c6eccf06e10c2f8e1529a44..95dc7c430ea238ea7ac7313dbe2126a698c8004d 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -1083,7 +1083,36 @@ void LayoutBox::middleClickAutoscroll(const IntPoint& sourcePoint) {
   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) {