Check for overlay scrollbars on all platforms

Make sure the code that fixes the scrollbar style only fires once.

Issue #2451

Author: marijnh

lib/codemirror.js

@@ -33,7 +33,6 @@
   var presto = /Opera\//.test(navigator.userAgent);
   var safari = /Apple Computer/.test(navigator.vendor);
   var khtml = /KHTML\//.test(navigator.userAgent);
-  var mac_geLion = /Mac OS X 1\d\D([7-9]|\d\d)\D/.test(navigator.userAgent);
   var mac_geMountainLion = /Mac OS X 1\d\D([8-9]|\d\d)\D/.test(navigator.userAgent);
   var phantom = /PhantomJS/.test(navigator.userAgent);
 
@@ -437,14 +436,18 @@
       d.gutterFiller.style.width = d.gutters.offsetWidth + "px";
     } else d.gutterFiller.style.display = "";
 
-    if (mac_geLion && scrollbarWidth(d.measure) === 0) {
-      d.scrollbarV.style.minWidth = d.scrollbarH.style.minHeight = mac_geMountainLion ? "18px" : "12px";
-      var barMouseDown = function(e) {
-        if (e_target(e) != d.scrollbarV && e_target(e) != d.scrollbarH)
-          operation(cm, onMouseDown)(e);
-      };
-      on(d.scrollbarV, "mousedown", barMouseDown);
-      on(d.scrollbarH, "mousedown", barMouseDown);
+    if (!cm.state.checkedOverlayScrollbar && measure.clientHeight > 0) {
+      if (scrollbarWidth(d.measure) === 0) {
+        var w = mac && !mac_geMountainLion ? "12px" : "18px";
+        d.scrollbarV.style.minWidth = d.scrollbarH.style.minHeight = w;
+        var barMouseDown = function(e) {
+          if (e_target(e) != d.scrollbarV && e_target(e) != d.scrollbarH)
+            operation(cm, onMouseDown)(e);
+        };
+        on(d.scrollbarV, "mousedown", barMouseDown);
+        on(d.scrollbarH, "mousedown", barMouseDown);
+      }
+      cm.state.checkedOverlayScrollbar = true;
     }
   }