Lookout for the scrollbarModes when force style change occurs.

third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.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@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 826 matching lines @@
         setHasVerticalScrollbar(false);
         return;
     }
 
     EOverflow overflowX = box().style()->overflowX();
     EOverflow overflowY = box().style()->overflowY();
 
     // To avoid doing a relayout in updateScrollbarsAfterLayout, we try to keep any automatic scrollbar that was already present.
     bool needsHorizontalScrollbar = (hasHorizontalScrollbar() && overflowDefinesAutomaticScrollbar(overflowX)) || overflowRequiresScrollbar(overflowX);
     bool needsVerticalScrollbar = (hasVerticalScrollbar() && overflowDefinesAutomaticScrollbar(overflowY)) || overflowRequiresScrollbar(overflowY);
+
+    // 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 destorying the scrollbars that was
+    // already present.
+    if (box().isLayoutView()) {
+        if (LocalFrame* frame = box().frame()) {
+            if (FrameView* frameView = frame->view()) {
+                ScrollbarMode hMode;
+                ScrollbarMode vMode;
+                frameView->calculateScrollbarModes(hMode, vMode);
+                if (hMode == ScrollbarAlwaysOn && !needsHorizontalScrollbar)
+                    needsHorizontalScrollbar = true;
+                if (vMode == ScrollbarAlwaysOn && !needsVerticalScrollbar)
+                    needsVerticalScrollbar = true;
+            }
+        }
+    }
+
     setHasHorizontalScrollbar(needsHorizontalScrollbar);
     setHasVerticalScrollbar(needsVerticalScrollbar);
 
     // With overflow: scroll, scrollbars are always visible but may be disabled.
     // When switching to another value, we need to re-enable them (see bug 11985).
     if (needsHorizontalScrollbar && oldStyle && oldStyle->overflowX() == OSCROLL && overflowX != OSCROLL) {
         ASSERT(hasHorizontalScrollbar());
         m_hBar->setEnabled(true);
     }
 
@@ skipping 627 matching lines @@
         return false;
 
     LocalFrame* frame = box().frame();
     if (!frame || !frame->isMainFrame() || !frame->settings())
         return false;
 
     return frame->settings()->viewportMetaEnabled();
 }
 
 } // namespace blink