Forgot What The HOtKey Was. It Isn't A Double Key.

D. It's my most-used shortcut in SONAR. Although technically speaking, it shifts between Track View and the Dock, so if you have for example the PRV in the dock, D will shift between Track View and PRV.
 
Note that D will toggle between the existing screen heights. However Shift+D will cause the selected view to go "full screen." They're both very useful shortcuts.