macOS Lost default behavior in XCode. Selecting to edit

Re: XCode 3.1.2 OS X 10.5.6

XCode Workspace Guide has the following
--begin
Opening Project Files

If you already have a project window open, you can open a file by
selecting it in the Groups & Files list in that window. If you have
the text editor pane open inside the project window, clicking the
filename opens the file in the editor. Otherwise, double-clicking
the file in the Groups & Files list opens the file in a separate
editor window.
--end

I found this all to be great a true until recently. I can
no longer simply select a file , say a *.m file , and have it
appear in the editor pane inside the project window. I am
forced to double click and have it open in a new window
or right click Open As -> Source Code File .

Xcode Preferences has
FileTypes: XML - Default Source File

selection of this type file under Group & Files also fails to open in
ProjWindowEditor

I have tried other projects, all of which worked in XCode in the
past. Here too , selection fails to load file.

Can you please advise this newbie how to fix this?
Thanks, Mark