I have considered the main branch as Martin's territory. It might have been practical to create a new branch for those non-related changes but too many branches is also messy.
I don't see problem there. How many commits there are in main branch those haven't been merged to d3d9branch ?
No new branches are needed, at least not in the Orbiter repository.
If someone external to the project finds and fixes an issue, or adds a new feature, they should create a PR
to the appropriate branch so those changes get incorporated into the project by internal personnel. The internal personnel can always make the change directly in a branch without a PR.
But it's actually a bit more than just OCD: a fix to, e.g., a memory leak in the VESSEL4 class (a "generic" problem, not related to feature work) that gets placed in a feature branch, means that all other branches will only get that fix when that branch gets merged back into the main branch. Yes, there's always copy-past and revision picking, but the best way is to put that fix in the main branch where it belongs, and then that, and other changes, get disseminated to the feature branches when they update themselves to the main.
In the end it all gets merged together and there should be no problems, but IMO things should be placed in the right places to minimize the probability of future issues.
You got to keep your worlds apart
BTW: thanks for the help above with the textures.