I do indeed have a portrait monitor for throwing terminals on to, and it's the worst place for making tearing visible.

I rarely use my external monitor, and I can't even remember the last time I had used it in portrait mode, so these comments made me realize there could be use cases (and not just hardware or drivers) that trigger tearing that I just hadn't encountered. But I just took a quick shot at it, and I didn't see tearing, at least not on YouTube, nor when moving a window around.

I'm on Wayland with Intel graphics.