Hey Naty, I think Robert's fix is a correct solution. And in fact there is something else wrong here since the HIG states that there should be a 12px border around the entirety of the window. As you stated, there is currently only a 4px border.
In all honesty, I'm not sure frames are really meant to be used side-by-side in this manner. So Robert's fix is more likely a work-around for using frames in a way they weren't intended to be used.
Hey Naty, I think Robert's fix is a correct solution. And in fact there is something else wrong here since the HIG states that there should be a 12px border around the entirety of the window. As you stated, there is currently only a 4px border.
In all honesty, I'm not sure frames are really meant to be used side-by-side in this manner. So Robert's fix is more likely a work-around for using frames in a way they weren't intended to be used.