The IDE renamed .PDE to _pde, and added .pde
I’ve tried saving the file as junk.pde
The IDE saved it as Junk_pde\junk_pde.pde.
(where Junk_pde is a NEW subdirectory made by the IDE.)
The IDE expects a sketch of a given name, like junk.pde, to be located in a directory of the same name without the .pde extension, like junk. There can be other files in that directory, with .cpp, .h, and .pde extensions.
Open the IDE, which will create a new sketch with today’s date as part of the name. Add a new tab. Give the file associated with that tab a name, like testing.h. Add another tab, with the name testing.cpp. Add a third tab, with the name functions.pde. Now, save the files, using File + save. Put the folder/sketch in C:\Temp.
Then, navigate to C:\Temp\sketch_<todays_date>. What is in that folder?
I just did this, and I have: