use translateZ(0) to avoid repaints of `#site` on scroll.

IPython/html/static/base/less/page.less

@@ -78,6 +78,12 b' body {'
 }
 
 #site {
+    // avoid repaints on size with translateZ(0)
+    -webkit-transform: translateZ(0);
+    -moz-transform: translateZ(0);
+    -ms-transform: translateZ(0);
+    -o-transform: translateZ(0);
+    transform: translateZ(0);
     width: 100%;
     display: none;
     .border-box-sizing();

IPython/html/static/style/style.min.css

@@ -8324,6 +8324,11 b' body {'
   height: 28px;
 }
 #site {
+  -webkit-transform: translateZ(0);
+  -moz-transform: translateZ(0);
+  -ms-transform: translateZ(0);
+  -o-transform: translateZ(0);
+  transform: translateZ(0);
   width: 100%;
   display: none;
   box-sizing: border-box;