It seems that JUCE components no longer respond to rollover and mouse wheel events in a window unless it is frontmost. On the previous version I was using, pre-Cocoa, these events did get responses even when the window was not frontmost. I’m guessing this is a Mac-only problem, but I’m currently working only on Mac so I’m not sure of that.
I updated JUCE from SVN today and built the demo app as well as the demo plugin without making any changes. Both the app and the plugin (in AuLab) show this behavior. Moving the mouse wheel over a slider in the demo app has no effect unless the window is frontmost.