TimEllis:
I just installed a fresh verison 1.8.9 of the IDE. I have some code that includes two .h files. I add the files using the sketch->add file. The .h files show up in the bar at the top and the code compiles fine. I made sure to save all of the files. When I close the IDE and reopen it the .h files do not show up. How can I be sure the .h files will reappear each time I open the IDE?
I just gave it a try, but was unable to reproduce the issue. After you add the .h file, select Sketch > Show Sketch Folder. Do you see the .h file in that folder?