Open the include folder of your GStudio installation, then open default.c. There you will find def_debug() which is the function in question to show the debug panel.
Your problem may be related to other things, for example when you press Alt+Tab to leave your game, (edit a shader, check your e-mails or whatever) then click your game window again, the engine will think that you are still pressing Alt. As a result, when pressing F11 only the following code will be executed:
void def_debug()
{
beep();
if(key_alt)
{
diag_status();
return;
}
...
}
