Visual Studio 2013 opens function's deceleration in the wrong menu

While debugging, for time to time, i use the F12 button to view function's definition / declaration. Usually, VS opens the needed file as a secondary tab in the main menu. Recently, not sure what i've done, it started opening the code in the lower menu, making the debugging process difficult to perform.

I think you've previously docked one or other window in an unintended position on screen. The visual up/down/left/right indicators shown when docking windows have different options for the down position and I suspect leads to this behaviour. Play around with the docking, or try the Reset Window Layout menu item under the Windows menu.