Add auto scrollbars if content would be visible without them.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintLayerScrollableArea.h"
 
 #include "core/frame/FrameView.h"
 #include "core/layout/LayoutBoxModelObject.h"
 #include "core/layout/LayoutTestHelper.h"
 #include "core/paint/PaintLayer.h"
@@ skipping 549 matching lines @@
       "<div id=\"scroller\"><div id=\"scrolled\"></div></div>");
   document().view()->updateAllLifecyclePhases();
   Element* scroller = document().getElementById("scroller");
   ASSERT_TRUE(scroller);
   PaintLayerScrollableArea* scrollableArea =
       toLayoutBoxModelObject(scroller->layoutObject())->getScrollableArea();
   ASSERT_TRUE(scrollableArea);
   scrollableArea->setScrollOffset(ScrollOffset(100, 0), ClampingScroll);
   EXPECT_EQ(scrollableArea->getScrollOffset().width(), 0);
 }
+
+TEST_F(PaintLayerScrollableAreaTest, ShowAutoScrollbarsForVisibleContent) {
+  RuntimeEnabledFeatures::setOverlayScrollbarsEnabled(false);
+  setBodyInnerHTML(
+      "<style>"
+      "#outerDiv {"
+      "  width: 15px;"
+      "  height: 100px;"
+      "  overflow-y: auto;"
+      "  overflow-x: hidden;"
+      "}"
+      "#innerDiv {"
+      "  height:300px;"
+      "  width: 1px;"
+      "}"
+      "</style>"
+      "<div id='outerDiv'>"
+      "  <div id='innerDiv'></div>"
+      "</div>");
+  document().view()->updateAllLifecyclePhases();
+  Element* outerDiv = document().getElementById("outerDiv");
+  ASSERT_TRUE(outerDiv);
+  outerDiv->layoutObject()->setNeedsLayout("test");
+  document().view()->updateAllLifecyclePhases();
+  PaintLayerScrollableArea* scrollableArea =
+      toLayoutBoxModelObject(outerDiv->layoutObject())->getScrollableArea();
+  ASSERT_TRUE(scrollableArea);
+  EXPECT_TRUE(scrollableArea->hasVerticalScrollbar());
 }
+}