Bug 1355211 - Scroll events should be prevented in all cases except when allowunderflowscroll is true and the scrollbar is hidden. r=enndeakin+6102

MozReview-Commit-ID: GXhf9bM6ZIo

--HG--
extra : rebase_source : 6599f20293c793a1dc10c4fce5467386cda878c8

toolkit/content/widgets/tree.xml

@@ -719,15 +719,15 @@
       </handler>
       <handler event="MozMousePixelScroll">
         <![CDATA[
-          if (this.getAttribute("allowunderflowscroll") != "true" &&
-              this.getAttribute("hidevscroll") != "true")
+          if (!(this.getAttribute("allowunderflowscroll") == "true" &&
+                this.getAttribute("hidevscroll") == "true"))
             event.preventDefault();
         ]]>
       </handler>
       <handler event="DOMMouseScroll">
         <![CDATA[
-          if (this.getAttribute("allowunderflowscroll") != "true" &&
-              this.getAttribute("hidevscroll") != "true")
+          if (!(this.getAttribute("allowunderflowscroll") == "true" &&
+                this.getAttribute("hidevscroll") == "true"))
             event.preventDefault();
 
           if (this._editingColumn)