As for Source List window, I have seen the following.
To adjust the width of Source List window, I drag ‘|||’ mark around bottom of this window and try to move it left or right.
When I push this mark to the most left side of window, then this window disappears, and I have only Record List window and Record Viewer.
For a moment, I fall in a panic state and search all the EF menu to restore this window. Finally, I restore it after mouse over on the most left side border of EF window and drag out to the right side a bit intentionally.
Of course, this may absolutely come from stupid action of new comer like me, but, is my action the right way to restore Source List window? Or, there is some EF menu that I missed over?
When I try to do the same stupid action in Mail.app, decreasing the width of mail-box window might be blocked to the minimum width determined by folder icon of Mailbox or plus mark.