Ok i did more testing and i can now reproduce the error with ANY delphi PAS file. Can someone test this to see if it its reproducible on your Delphi as well.
It all stems from right clicking any open .PAS file and selecting 'New Edit Window'.
Once you have the source file open in a second edit window, Compile your App and let run, then close your app and return to the IDE.
Now leave the second edit window open and close your project, close delphi, and restart delphi.
Once you are back in Delphi, open your project, then from the open tabs, close your PAS file that is open in the second edit window.
Now if you try and double click that PAS file in the Project window it won't open the file any more. (BUG)
How you can gain the functionality back is to open your Project.dsk file and in the [Modules] section, find the line that has your stuck pas file and delete that line.
Reopen delphi and you can now open your PAS file again.
So there is a problem with the IDE and having a new edit window open. Its like the PAS file thats associated with the new edit window gets stuck. So when you try and open it Delphi thinks its already open but you never get the interface.