The plugInViewWithArguments: API sends dictionary of plugin arguments. One of the parameters is WebPlugInBaseURLKey, which is a key that represents the base URL of the document containing the plug-in's view. Instead what we're actually getting is the source URL of the plug-in resource.
<rdar://problem/7673157>
Created attachment 49159 [details] proposed fix
http://trac.webkit.org/changeset/55063
Had to roll out the fix in <http://trac.webkit.org/changeset/58120>. <rdar://problem/7896604>
Mass closing plug-in bugs, as plug-in support has been removed from WebKit. Please comment and/or reopen if this still affects WebKit in some way.