How about-its a just a fairly useless feature that even if it did work properly(which it does not), would at best be, of marginal utility. Once I mastered the art of making my own designs, and disabled the boxy looking stock designs, I found the game a lot more fun. The few times I tried to use the 'focus' button, all I learned was i was spending too much time focusing on something that didn't really do anything very useful.
So I stopped trying ages ago.
My 2cents? Ignore it-make your own ships that appear in a linear fashion-building on previous designs(your own). Its much more fun-and it works. My designs hold there own even against superior numbers because the game frankly, seldom maxes the tech trees. The few times the AI has, well, damn, there ships can be tough, but that seldom happens. Even with numerical superiority , even my med and L class fleets do very well most times. I never change my designs armor types even to match the AIs preferred weapons-there is seldom any need to.