I use Geany also for work and there we happily accept editor-specific files in the repos if it helps our engineers to get their job done.
Good employer :-).
This is especially true for Windows and OSX stuff where we commit the whole VS and Xcode project metadata (multiple files).
Yes, but this is a necessity - in XCode for instance the project itself defines how to build it and I, too, have it committed for the macOS launcher of Geany here:
https://github.com/geany/geany-osx/tree/master/Launcher/geany/geany.xcodepro...
Geany, on the other hand, doesn't define the build process itself and the way I see it, it's a bunch of personal preferences of how to run the build process or launch your project and these may differ user to user. I gave the example with `make -j 2` vs `make -j 9` (and yeah, this will be gone if we move to meson - kudos for your work in this area btw.), plugin settings and I think that nobody wants to see the Geany project file as modified in git permanently if he customizes it to his needs.
Or to tell it in another way in terms of XCode:
xcode project ~~ autotools or meson build system and is committed xcuserdata directory ~~ Geany project - personal settings
Since the way I see Geany projects is personal preferences only, I don't see much need for the project/session split (and having to deal with the various problems discussed above).