Where an ancestor has overflow style, a fixed child needs to be composited.

LayoutTests/platform/mac/compositing/layer-creation/fixed-position-in-fixed-overflow-expected.txt

-This layer should not have backing store.
-This layer should not have backing store.
 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
-          "position": [8, 8],
-          "bounds": [342, 244],
+          "position": [8, 13],
+          "bounds": [800, 600],
           "drawsContent": true,
           "children": [
             {
-              "position": [1, 1],
-              "bounds": [340, 242],
+              "bounds": [785, 600],
               "children": [
                 {
-                  "position": [20, 40],
-                  "bounds": [342, 182],
+                  "bounds": [785, 1000],
                   "drawsContent": true,
                   "children": [
                     {
-                      "position": [1, 1],
-                      "bounds": [340, 180],
                       "children": [
                         {
-                          "position": [30, 50],
+                          "position": [192, 0],
                           "bounds": [100, 100],
                           "contentsOpaque": true,
                           "drawsContent": true,
-                          "backgroundColor": "#C0C0C0"
+                          "backgroundColor": "#FFFF00"
                         }
                       ]
                     }
                   ]
                 }
               ]
+            },
+            {
+              "children": [
+                {
+                  "position": [785, 0],
+                  "bounds": [15, 600]
+                }
+              ]
             }
           ]
         }
       ]
     }
   ]
 }