Fix transitions to default fill-outline-color

src/style/properties.js

@@ -547,7 +547,7 @@ class DataDrivenProperty<T> implements Property<T, PossiblyEvaluatedPropertyValu
         }
 
         // Special case hack solely for fill-outline-color.
-        if (a.value.value === undefined || a.value.value === undefined)
+        if (a.value.value === undefined || b.value.value === undefined)
             return (undefined: any);
 
         const interp: ?(a: T, b: T, t: number) => T = (interpolate: any)[this.specification.type];

test/integration/render-tests/regressions/mapbox-gl-js#5953/style.json

@@ -0,0 +1,66 @@
+{
+  "version": 8,
+  "metadata": {
+    "test": {
+      "width": 64,
+      "height": 64,
+      "diff": 0.001,
+      "operations": [
+        [
+          "wait"
+        ],
+        [
+          "setPaintProperty",
+          "fill",
+          "fill-outline-color",
+          null
+        ],
+        [
+          "wait"
+        ]
+      ]
+    }
+  },
+  "sources": {
+    "geojson": {
+      "type": "geojson",
+      "data": {
+        "type": "Polygon",
+        "coordinates": [
+          [
+            [
+              -10,
+              -10
+            ],
+            [
+              -10,
+              10
+            ],
+            [
+              10,
+              10
+            ],
+            [
+              10,
+              -10
+            ],
+            [
+              -10,
+              -10
+            ]
+          ]
+        ]
+      }
+    }
+  },
+  "layers": [
+    {
+      "id": "fill",
+      "type": "fill",
+      "source": "geojson",
+      "paint": {
+        "fill-outline-color": "black"
+      }
+    }
+  ]
+}