I'm using IDE 2.1.0 under Win 10 with a large project of more than 20 files. I was working with the latest version of the project, and decided to load/compile/upload an earlier copy of the same project, but the earlier version was without a specific header file (MyConfigurationFile.h). I loaded up the earlier version, it compiled and ran as expected. I closed the older project, then used File Explorer to copy the new header file into the previous version of the project. Note, the IDE is still open, but only with the current version.
I now reload the older version, the project which I expected would now have the new header file in it. It loads fine, but the file tabs across the top do not show the new header file. I closed both the older and current project files and close the IDE. I then restart the IDE, which boots with the latest project file in memory. I then load the older project which now has the new header file, but the file tabs above the source window still does not show the new header file.
However, if I try to compile the older project, I get error messages that can only be generated if the compiler is reading the new file (i.e., duplicate definitions on symbolic constants that were defined in the original header file, but moved to the new one). So, I looked at the project's file list using the ellipsis on the upper-right edge of the Source Code window (because the file list is too long to show in the file tabs above the Source Code window), and, sure enough, the new header file is in the list. If I click on the new header file entry, the IDE does open it up, but the file name has been appended to the tab list. That is, it's not in alpha order in the tab list.
Truthfully, I have not edited the old project errors or saved the project for fear of messing up the old version of the project. I can continue to work on the new file, but I eventually want to modify the old project.