Fixed with the new string size optimisation: the cache value was returned by reference and something in the CPButton path for image and text buttons changed it causing text to become misaligned. A defensive copy avoids this problem.

AppKit/CPStringDrawing.j

@@ -60,7 +60,7 @@ var CPStringSizeWithFontInWidthCache = {};
         CPStringSizeWithFontInWidthCache[cacheKey] = size;
     }
 
-    return size;
+    return CGSizeMakeCopy(size);
 }