Fixed: sheets should not be resizable from the top

Previously, sheets could be resized from the top, which would destroy the illusion of being attached to the top of the parent window.

With this commit, sheets are prevented from being resized by the user from the top edge.

AppKit/CPWindow/_CPWindowView.j

@@ -245,6 +245,8 @@ _CPWindowViewResizeSlop = 3;
         rects are the same size. So to save calculations, we can just create
         3 rects and move them around to do hit testing. Start with the corners
         and then do left/right and top/bottom.
+
+        NOTE: If the window is a sheet, the top is never eligible for resizing.
     */
     var wind = [self window],
         frame = [wind frame],
@@ -252,7 +254,8 @@ _CPWindowViewResizeSlop = 3;
         minSize = [wind minSize],
         maxSize = [wind maxSize],
         isFixedWidth = minSize.width === maxSize.width,
-        isFixedHeight = minSize.height === maxSize.height;
+        isFixedHeight = minSize.height === maxSize.height,
+        isSheet = wind._isSheet;
 
     if (isFixedWidth)
     {
@@ -262,7 +265,7 @@ _CPWindowViewResizeSlop = 3;
                            _CPWindowViewResizeSlop * 2);
 
         if (_CGRectContainsPoint(rect, aPoint))
-            return _CPWindowViewResizeRegionTop;
+            return isSheet ? _CPWindowViewResizeRegionNone : _CPWindowViewResizeRegionTop;
 
         rect.origin.y = _CGRectGetMaxY(frame) - _CPWindowViewResizeSlop;
 
@@ -292,12 +295,12 @@ _CPWindowViewResizeSlop = 3;
                            _CPWindowViewCornerResizeRectWidth + _CPWindowViewResizeSlop);
 
         if (_CGRectContainsPoint(rect, aPoint))
-            return _CPWindowViewResizeRegionTopLeft;
+            return isSheet ? _CPWindowViewResizeRegionNone : _CPWindowViewResizeRegionTopLeft;
 
         rect.origin.x = _CGRectGetMaxX(frame) - _CPWindowViewCornerResizeRectWidth;
 
         if (_CGRectContainsPoint(rect, aPoint))
-            return _CPWindowViewResizeRegionTopRight;
+            return isSheet ? _CPWindowViewResizeRegionNone : _CPWindowViewResizeRegionTopRight;
 
         rect.origin.y = _CGRectGetMaxY(frame) - _CPWindowViewCornerResizeRectWidth;
 
@@ -328,7 +331,7 @@ _CPWindowViewResizeSlop = 3;
                            _CPWindowViewResizeSlop * 2);
 
         if (_CGRectContainsPoint(rect, aPoint))
-            return _CPWindowViewResizeRegionTop;
+            return isSheet ? _CPWindowViewResizeRegionNone : _CPWindowViewResizeRegionTop;
 
         rect.origin.y = _CGRectGetMaxY(frame) - _CPWindowViewResizeSlop;