A yeaa, it works, thanks. TabIndex had really strange default value.
Shift+TAB would be great, but I think numpad arrows are even higher priority. Little bit unexpected for the user they don't work. I always use numpad arrows because regular arrows keys are so small and uncomfortable on a laptop. I would never buy a laptop without a numpad because of this. I guess there might be other users doing the same thing.
In case you do any improvement on this, these are other focus changing keys which are important:
Ctrl+TAB - next tab page in TabControl
same with shift - previous tab page
I guess Home, End, PgUp, PgDown could work too, but this is going advanced