I am sure, this flaw was reported before, but maybe it’s time to think about this again? 
On OSX, if I open an AU or VST in the external view, and then open a file requester within the plugin, the plugin will loose keyboard focus. This happens with a bunch of plugins, if not all. I have to click then on Renoise main window and then on the plugin window to get keyboard piano focus again…
I am no OSX programmer, but is there really no system event which is fired when a file requester closes, so Renoise could hook on this event and rebuild the focus again? If you work only with the computer’s keyboard, this would reduce workflow irritation a lot.