What is the threshold for indirect patent infringement? For instance, I don't fear being sued personally for using Microsoft office (which I'm sure infringes on something).
Not a lawyer, but as far as I understand there's no threshold or legal distinction between direct and indirect. It's all direct.
There's the angle of "how much $ can I extract" that comes into play, which is based on damages incurred from infringement. If you're not making money or preventing the patent holder from making money, you're not worth harassing.
It’s just an inefficient way to sue people and Microsoft would likely step in.
But it’s common to get sued for using functionality in open source software.