DevTools: promote large views with overflow to layers in order to not repaint on scroll.

Source/devtools/front_end/elementsPanel.css

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Anthony Ricaud <rik@webkit.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 13 matching lines @@
  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
  * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #elements-content {
     flex: 1 1;
     overflow: auto;
     padding-left: 0;
+    -webkit-transform: translateZ(0);

[caseq, 2014/03/20 09:03:22]

Is this one really necessary?

 }
 
 #elements-crumbs {
     flex: 0 0 19px;
     background-color: rgb(236, 236, 236);
     border-top: 1px solid #ccc;
     overflow: hidden;
 }
 
 #elements-content > ol {
     display: inline-block;
     min-height: 100%;
+    -webkit-transform: translateZ(0);
 }
 
 #elements-content .editing {
     margin-left: 8px;
 }
 
 #elements-content .elements-gutter-decoration {
     position: absolute;
     left: 1px;
     margin-top: 2px;
@@ skipping 504 matching lines @@
 .image-preview-container img {
     margin: 2px auto;
     max-width: 100px;
     max-height: 100px;
     background-image: url(Images/checker.png);
     -webkit-user-select: text;
     -webkit-user-drag: auto;
 }
 
 .sidebar-pane.composite {
-    overflow: hidden;
     position: absolute;
 }
 
 .sidebar-pane.composite > .body {
     height: 100%;
 }
 
 .sidebar-pane.composite .metrics {
     border-bottom: 1px solid rgb(64%, 64%, 64%);
     height: 206px;
@@ skipping 104 matching lines @@
 
 .styles-clipboard-only {
     display: inline-block;
     width: 0;
     opacity: 0;
 }
 
 li.child-editing .styles-clipboard-only {
     display: none;
 }