Interesting, so you commit things that are different from the code that is sitting in your working directory at the time of the commit?
Later on, if I notice that I messed something up, I'll create a fixup commit and do an interactive rebase to squash it back into the correct commit.
It should be a crime for people to blindly stage everything. Cleaning up after my colleagues easily falls under the "lost cause" umbrella.
If I really care about things being fully correct at each commit, I'll do an interactive rebase and just stop and test on each commit.
Often they're disjoint enough that I'm confident that if the end result works, the intermediate commit works, as the sibling commenter mentioned. (Even if not, on many teams it all gets squashed anyway, the intermediate commit message just makes the Pull Request easier to review.)
I can also often easily comment out the uncommitted stuff.
Finally, if I'm really worried about it, after creating the intermediate commit I can just `git stash`, test it, then `git stash pop` and keep working.