Fix performance regression and bug with hide_scrollbars setting.

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 673 matching lines @@
 
     // overflow:scroll should just enable/disable.
     if (box().style()->overflowX() == OverflowScroll && horizontalScrollbar())
       horizontalScrollbar()->setEnabled(hasHorizontalOverflow);
     if (box().style()->overflowY() == OverflowScroll && verticalScrollbar())
       verticalScrollbar()->setEnabled(hasVerticalOverflow);
   }
 
   // We need to layout again if scrollbars are added or removed by
   // overflow:auto, or by changing between native and custom.
+  DCHECK(box().frame()->settings());
   bool horizontalScrollbarShouldChange =
-      (box().hasAutoHorizontalScrollbar() &&
-       (hasHorizontalScrollbar() != shouldHaveAutoHorizontalScrollbar)) ||
-      (box().style()->overflowX() == OverflowScroll && !horizontalScrollbar());
+      ((box().hasAutoHorizontalScrollbar() &&
+        (hasHorizontalScrollbar() != shouldHaveAutoHorizontalScrollbar)) ||
+       (box().style()->overflowX() == OverflowScroll &&
+        !horizontalScrollbar())) &&
+      (!box().frame()->settings()->hideScrollbars() ||
+       hasHorizontalScrollbar());
   bool verticalScrollbarShouldChange =
-      (box().hasAutoVerticalScrollbar() &&
-       (hasVerticalScrollbar() != shouldHaveAutoVerticalScrollbar)) ||
-      (box().style()->overflowY() == OverflowScroll && !verticalScrollbar());
+      ((box().hasAutoVerticalScrollbar() &&
+        (hasVerticalScrollbar() != shouldHaveAutoVerticalScrollbar)) ||
+       (box().style()->overflowY() == OverflowScroll &&
+        !verticalScrollbar())) &&
+      (!box().frame()->settings()->hideScrollbars() || hasVerticalScrollbar());
   bool scrollbarsWillChange =
       !scrollbarsAreFrozen && !visualViewportSuppliesScrollbars() &&
       (horizontalScrollbarShouldChange || verticalScrollbarShouldChange);
 
   if (scrollbarsWillChange) {
     bool hadHorizontalScrollbar = hasHorizontalScrollbar();
     bool hadVerticalScrollbar = hasVerticalScrollbar();
     if (box().hasAutoHorizontalScrollbar())
       setHasHorizontalScrollbar(shouldHaveAutoHorizontalScrollbar);
     else if (box().style()->overflowX() == OverflowScroll)
@@ skipping 219 matching lines @@
           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 &&
+  if (hasHorizontalScrollbar() && oldStyle &&
       oldStyle->overflowX() == OverflowScroll && overflowX != OverflowScroll) {
-    ASSERT(hasHorizontalScrollbar());
     horizontalScrollbar()->setEnabled(true);
   }
 
-  if (needsVerticalScrollbar && oldStyle &&
+  if (hasVerticalScrollbar() && oldStyle &&
       oldStyle->overflowY() == OverflowScroll && overflowY != OverflowScroll) {
-    ASSERT(hasVerticalScrollbar());
     verticalScrollbar()->setEnabled(true);
   }
 
   // FIXME: Need to detect a swap from custom to native scrollbars (and vice
   // versa).
   if (horizontalScrollbar())
     horizontalScrollbar()->styleChanged();
   if (verticalScrollbar())
     verticalScrollbar()->styleChanged();
 
@@ skipping 1020 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollPositionClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollPositionsAfterLayout();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink