My Desk is a Border War

I have a work issue. Here's the deal: I joined this team about 4 months ago. The desk configuration here is a "modern open floor-plan" - meaning that instead of square cubes, we have long rows of desks, divided in pairs by low walls. 

So, essentially, you have a "roommate" with which you share a section of desk. I was given a desk next to an established team member. Each pair of desks is pretty clearly split into two by a divided pinboard/cube wall. Here's what happening with our desk:

Note: This secret photo was captured after I passive-aggressively shoved everything as far as I could back to her side of the desk.

My roommate has her computer configured to take the maximum amount of desk space, which leads to a slow and expanding encroachment upon my half of the desk. Why does this make me feel so stabby? I realize that roommate was not sharing the desk with anyone before I showed up here. But isn't it kind of rude to encroach on a visible border, and further, isn't it just mean to watch me eat my lunch on top of my keyboard and knock my phone off the hook when trying to navigate my crowded desktop? Or, am I the Dwight Schrute of this desk row?