The horizontal red bar shows which window will scroll when you press the Up/Down Arrow and PageUp/Down keys or the scroll wheel on your mouse. To flip the focus to the opposite window, press Left or Right Arrow key.
Go to Top or Bottom of Window
Press Home or End to go to the top or bottom of whichever window is under the red bar.
The Red Bar
The focus is currently the text window (the definitions window). Press Left or Right Arrow key to go to the opposite window.