Function mapper performance check requires null check for some props

Some properties, like `control-point-distances`, do not have non-null default values to fall back on.  These properties are typically override properties:  The property overrides the behaviour of another one and the `null` value indicates that the behaviour override is inactive.  Because most properties do have a default value, it's very subtle.  The performance enhancement in #2239 assumes that all properties have a default value, missing the null check.  This causes the issue.

Ref : When applying a function mapper to control-point-distances, eleProp is null in applyContextStyle() #2423

Ref : Optimizations for mapped style properties #2239

src/style/apply.js

@@ -210,6 +210,7 @@ styfn.applyContextStyle = function( cxtMeta, cxtStyle, ele ){
     // save cycles when a mapped context prop doesn't need to be applied
     if(
       cxtProp.mapped === types.fn // context prop is function mapper
+      && eleProp != null // some props can be null even by default (e.g. a prop that overrides another one)
       && eleProp.mapping != null // ele prop is a concrete value from from a mapper
       && eleProp.mapping.value === cxtProp.value // the current prop on the ele is a flat prop value for the function mapper
     ){ // NB don't write to cxtProp, as it's shared among eles (stored in stylesheet)

test/collection-style.js

@@ -5,12 +5,6 @@ describe('Collection style', function(){
 
   var cy;
 
-  var useFn = function( fn ){
-    return function( arg ){
-      return fn( arg );
-    };
-  };
-
   // test setup
   beforeEach(function(){
     cy = cytoscape({
@@ -33,7 +27,7 @@ describe('Collection style', function(){
         {
           selector: '#n1',
           style: {
-            label: useFn(function(){ return 'n1'; }),
+            label: function(){ return 'n1'; },
             width: 20,
             'background-image': ['/test/image.png', '/test/image2.png'],
             opacity: 0.5
@@ -43,7 +37,7 @@ describe('Collection style', function(){
         {
           selector: '#n2',
           style: {
-            label: useFn(function(){ return 'n2'; })
+            label: function(){ return 'n2'; }
           }
         },
 
@@ -585,6 +579,23 @@ describe('Collection style', function(){
         expect(edge.isBundledBezier(), edge.id()).to.be.false;
       });
     });
+
+    it('ele.style() reads OK for mapped override prop', function(){
+      cy.style().fromJson([
+        {
+          selector: '#n1n2',
+          style: {
+            'curve-style': 'unbundled-bezier',
+            'control-point-distances': function(ele){ return [32, 128]; },
+            'control-point-weights': [0.5, 0.75]
+          }
+        }
+      ]).update();
+
+      var d = cy.$('#n1n2').numericStyle('control-point-distances');
+
+      expect(d, 'control-point-distances').to.deep.equal([32, 128]);
+    });
   });
 
   describe('eles.addClass() etc', function(){