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Mount openide-io.jar from modules/autoload. Now update the code completions for it. A dialog pops up with the following warning: Class "IOProvider" claims to exist in package "", yet it is mounted under package "org.openide.windows". The package "org.openide.windows" will not be updated." ...and the code completion db is not updated. This problem is not limited to that particular jar.
The code completion functionality will be rewritten to use MDR. It is an editor issue. For now, you should turn the automatical updates of the codecompletion off by going to Tools->Options-->Editing->Editor Settings->Java Editor and setting "Update Code Completion Database After Mounting" property in Expert tab to "Never".
Code completion has been rewritten to use Java/MDR.