On Mac OS X, WebKit only recently implemented NPN_PluginThreadAsyncCall. See https://bugs.webkit.org/show_bug.cgi?id=63868. I suspect this is why a number of test suite failures occur in Safari, even after fixing the invalid HTML.

We will need to work around this. It can be done pretty easily by using a block to call the callback on the main thread, which is what NPN_PluginThreadAsyncCall would likely do anyway.

Much of the plugin implementation is relying on this, so it should fix a number of problems.