That's a designed behavior. Couple of solutions you might try. See which you prefer. After you use the F key to do the edit, press the Q key to make the viewer active. The other way, which I use when I'm cutting selects from a longish clip the viewer is to drag to the canvas edit overlay to overwrite. That leaves the viewer the active window. Your choice. There is no way to change the programmed behavior.
All the best,
Tom