You're seeing - perhaps for the first time since you've been using a Mac - something that's been a part of the Mac OS since the early 1990's (System 7.1 era).
In applications that do not support input methods at all, the Mac OS will, on the application's behalf, collect the input in this floating "bottomline input" window.
In these applications, you probably want to use the Suggest Spelling window, and you probably won't be able to use completion. Still, you may be able to somehow adjust to working with the bottomline window - your mileage may vary. You can move and resize this window, so positioning it on your screen in a way that it won't interfere with what you see in your document and yet still be close enough to where you are typing may be workable. Try it and decide for yourself.
This is documented in Spell Catcher's online Help Book. Choose Spell Catcher Help from the Help menu in the Spell Catcher application, view the end of the section on the Pop-Up Suggestions List (scroll to the very bottom).