This was partially my fault. I should've known better than to stay in his blind spot that long. But the light was red ahead and I didn't feel like speeding up or slowing down dramatically. So I just "coasted" to the traffic light. All the signs were there.
1. He left his left blinker on after switching lanes.
2. He was slow to move after the light turned green.
Should've recognized those as signs of an inattentive driver. I didn't have to face palm him. His passenger did that already.
Link to original page on YouTube.