SelectionAutoscroll should not happen in user-select:none.

third_party/WebKit/Source/core/input/MouseEventManager.cpp

Index: third_party/WebKit/Source/core/input/MouseEventManager.cpp
diff --git a/third_party/WebKit/Source/core/input/MouseEventManager.cpp b/third_party/WebKit/Source/core/input/MouseEventManager.cpp
index c38ab2fcb8db66b6adc03875a6e1446fdffb617a..aa178a3276abcdc653bc93d979c86176c10dbe98 100644
--- a/third_party/WebKit/Source/core/input/MouseEventManager.cpp
+++ b/third_party/WebKit/Source/core/input/MouseEventManager.cpp
@@ -733,7 +733,8 @@ WebInputEventResult MouseEventManager::handleMouseDraggedEvent(
   m_mouseDownMayStartDrag = false;
 
   if (m_mouseDownMayStartAutoscroll &&
-      !m_scrollManager->middleClickAutoscrollInProgress()) {
+      !m_scrollManager->middleClickAutoscrollInProgress() &&
+      layoutObject->isSelectable()) {

[bokan, 2016/12/02 17:32:38]

Selection is created/added by the call below to handleMouseDraggedEvent right?
I'm wondering, would it work if we moved this scrolling to happen after that and
then just check whether or not we have a selection? I'm worried that this check
is too simplistic and there might be all sorts of other cases where we can't
select even though the layout object might be selectable. It would be better if
we can just rely on the selection system to worry about all the edge cases and
use its output to decide whether we should scroll.

[sunyunjia, 2016/12/04 22:46:43]

Done.

     if (AutoscrollController* controller =
             m_scrollManager->autoscrollController()) {
       controller->startAutoscrollForSelection(layoutObject);