Add auto scrollbars if content would be visible without them.

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 1190 matching lines @@
     return;
   }
 
   needsHorizontalScrollbar = box().scrollsOverflowX();
   needsVerticalScrollbar = box().scrollsOverflowY();
 
   // Don't add auto scrollbars if the box contents aren't visible.
   if (box().hasAutoHorizontalScrollbar()) {
     if (option == ForbidAddingAutoBars)
       needsHorizontalScrollbar &= hasHorizontalScrollbar();
-    needsHorizontalScrollbar &= box().isRooted() &&
-                                this->hasHorizontalOverflow() &&
-                                box().pixelSnappedClientHeight();
+    needsHorizontalScrollbar &=
+        box().isRooted() && this->hasHorizontalOverflow() &&
+        box().pixelSnappedClientHeight() + box().horizontalScrollbarHeight() >
+            0;
   }
 
   if (box().hasAutoVerticalScrollbar()) {
     if (option == ForbidAddingAutoBars)
       needsVerticalScrollbar &= hasVerticalScrollbar();
-    needsVerticalScrollbar &= box().isRooted() && this->hasVerticalOverflow() &&
-                              box().pixelSnappedClientWidth();
+    needsVerticalScrollbar &=
+        box().isRooted() && this->hasVerticalOverflow() &&
+        box().pixelSnappedClientWidth() + box().verticalScrollbarWidth() > 0;
   }
 
   // Look for the scrollbarModes and reset the needs Horizontal & vertical
   // Scrollbar values based on scrollbarModes, as during force style change
   // StyleResolver::styleForDocument returns documentStyle with no overflow
   // values, due to which we are destroying the scrollbars that were already
   // present.
   if (box().isLayoutView()) {
     if (LocalFrame* frame = box().frame()) {
       if (FrameView* frameView = frame->view()) {
@@ skipping 903 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetAfterOverflowChange();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink