Currently we only composite fixed position elements if we are on High DPI devices. This fix allows …

LayoutTests/fast/repaint/resources/default.css

 html {
     overflow: hidden;
 }
 
 div, td {
     width: 100px;
     height: 100px;
 }
 
 iframe {
@@ skipping 10 matching lines @@
 }
 
 .relative {
     position: relative;
 }
 
 .red {
     background-color: red;
 }
 
+.red-alpha {
+    background-color: rgba(255, 0, 0, 0.99);
+}
+
 .blue {
     background-color: blue;
 }
 
 .green {
     background-color: green;
 }
 
 .yellow {
     background-color: yellow;
 }
 
 .rotated {
     transform: rotate(45deg);
 }
 
 .clipped {
     overflow: hidden;
 }
 
 .reflected {
     -webkit-box-reflect: below 0px;
 }