When you've got many files open in the code editor of Delphi, there's another handy but undocumented shortcut: pressing Ctrl+Alt+F12 opens the drop-down on the top-right corner of the editor, listing all open files.
Use incremental search (i.e. just start typing) to select the file you want and then press Enter to activate that editor tab!
That's just one of the many editor shortcuts. Check out the Delphi Wiki for more.