I don't think many people use calibre as a way to read their ebooks, so you're likely to find problems that just haven't been noticed before. The missing search bar in full screen mode is an exemplary example of the kind of thing you'll find.
But that doesn't mean you shouldn't report the problems you find, or ask for features you see as missing.
If you want to reort a definite bug, the best way is through calibre bug reporting database which you can find from
this link.