Closes #429. Doesn't necessarily address the root cause however.

The problem stems from a bug in Safari which is allowing callbacks
to interrupt running code (which should be impossible in JavaScript).
The change essentially makes it so that in the case where this happens,
which is rare, we'll end up accidentally creating an extra iframe.
This isn't ideal, but it also isn't harmful, versus the current
situation where code will simply break.

AppKit/Platform/DOM/CPPlatform.j

@@ -21,6 +21,7 @@
  */
 
 CPPlatformDidClearBodyElementNotification   = @"CPPlatformDidClearBodyElementNotification";
+CPPlatformWillClearBodyElementNotification  = @"CPPlatformWillClearBodyElementNotification";
 
 var screenNeedsInitialization   = NO,
     mainBodyElement = nil;
@@ -91,6 +92,10 @@ var screenNeedsInitialization   = NO,
 
     screenNeedsInitialization = NO;
 
+    [[CPNotificationCenter defaultCenter]
+        postNotificationName:CPPlatformWillClearBodyElementNotification
+                      object:self];
+
     var bodyElement = [self mainBodyElement];
 
     // Get rid of any of the original contents of the page.

AppKit/Platform/DOM/CPPlatformString.j

@@ -22,8 +22,8 @@
 
 #include "../CoreGraphics/CGGeometry.h"
 
-
 var DOMSpanElement      = nil,
+    DOMIFrameElement    = nil,
     DefaultFont         = nil;
 
 @implementation CPPlatformString : CPBasePlatformString
@@ -39,11 +39,20 @@ var DOMSpanElement      = nil,
            selector:@selector(platformDidClearBodyElement:)
                name:CPPlatformDidClearBodyElementNotification
              object:CPPlatform];
+
+    [[CPNotificationCenter defaultCenter]
+        addObserver:self
+           selector:@selector(platformWillClearBodyElement:)
+               name:CPPlatformWillClearBodyElementNotification
+             object:CPPlatform];
 }
 
 + (void)createDOMElements
 {
-    var DOMIFrameElement = document.createElement("iframe");
+    if (DOMIFrameElement)
+        return;
+
+    DOMIFrameElement = document.createElement("iframe");
     // necessary for Safari caching bug:
     DOMIFrameElement.name = "iframe_" + FLOOR(RAND() * 10000);
     DOMIFrameElement.style.position = "absolute";
@@ -80,11 +89,27 @@ var DOMSpanElement      = nil,
     DOMDivElement.appendChild(DOMSpanElement);
 }
 
++ (void)removeDOMElements
+{
+    var iframe = DOMIFrameElement,
+        bodyElement = [CPPlatform mainBodyElement];
+
+    DOMSpanElement = nil;
+    DOMIFrameElement = nil;
+
+    bodyElement.removeChild(iframe);
+}
+
 + (void)platformDidClearBodyElement:(CPNotification)aNotification
 {
     [self createDOMElements];
 }
 
++ (void)platformWillClearBodyElement:(CPNotification)aNotification
+{
+    [self removeDOMElements];
+}
+
 + (CGSize)sizeOfString:(CPString)aString withFont:(CPFont)aFont forWidth:(float)aWidth
 {
     if (!aFont)
@@ -95,35 +120,30 @@ var DOMSpanElement      = nil,
         aFont = DefaultFont;
     }
 
+    if (!DOMIFrameElement)
+        [self createDOMElements];
+
     var style = DOMSpanElement.style;
 
-    if (aWidth === NULL)
+    if (!aWidth)
     {
         style.width = "";
         style.whiteSpace = "pre";
     }
-    
     else
     {
         style.width = ROUND(aWidth) + "px";
-        
-        if (document.attachEvent)
-            style.wordWrap = "break-word";
-        
-        else
-        {
-            style.whiteSpace = "-o-pre-wrap";
-            style.whiteSpace = "-pre-wrap";
-            style.whiteSpace = "-moz-pre-wrap";
-            style.whiteSpace = "pre-wrap";
-        }
+        style.wordWrap = "break-word";
+        style.whiteSpace = "-o-pre-wrap";
+        style.whiteSpace = "-pre-wrap";
+        style.whiteSpace = "-moz-pre-wrap";
+        style.whiteSpace = "pre-wrap";
     }
 
     style.font = [aFont cssString];
 
     if (CPFeatureIsCompatible(CPJavascriptInnerTextFeature))
         DOMSpanElement.innerText = aString;
-
     else if (CPFeatureIsCompatible(CPJavascriptTextContentFeature))
         DOMSpanElement.textContent = aString;