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NPAPI Questions

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These are questions that bsmedberg found while trying to implement remoting for pieces of the NPAPI. I'd like to have tests and docs for these once the answers are known:

NPN_GetURL and family have an optional target; does the behavior change if the target is null versus the empty string ""? 
Yes. If the parameter is NULL, the data is sent back to the plugin as a stream. If the parameter is "", the current window is replaced with the URL.
NPStream has a url field; are there cases where this may be null? If so, would making it the empty string change the behavior?
Do streams receieve a NPP_Destroy message before the associated instance is killed with NPP_Destroy? What reason code is given?
Do url notifications receieve a NPP_URLNotify message before the associated instance is killed with NPP_Destroy? What reason code is given?
Are there instances where making calls using NPRuntime is disallowed? (e.g. re-entering script from NPP_New can cause crashes in gecko)
After NPP_NewStream is called, who owns the notifyData field of NPStream? 
Flash uses this field to store scratch data. Plugins should be allowed to store data and modify this field after the initial call to NPP_NewStream.
npruntime.h seems to suggest that an NPClass' invalidate method is only called by the runtime, yet we seem to have code that handles the plugin calling it on browser-provided objects. What's the deal?
May NPClass.setProperty be null? What is the expected behavior? Will the host throw an exception if webpage code attempts to set a property, or will it set a custom propery?
May NPClass.invokeDefault be null?
May NPClass.invoke be null? What is the expected behavior? Will attempting to call pluginobject.somefunction() fall back to calling .getProperty and .invokeDefault in sequence?
Will the plugin host modify the NPClass structure passed in to NPN_CreateObject (e.g. to provide default behavior for null class pointers)? If not, can this be made a const NPClass*?