I'm new to Blender and am trying to understand UV mapping using this tutorial: https://www.youtube.com/watch?v=XeBUfMKKZDo
I modeled a simple staircase by starting with a cube. First I subdivided two adjacent edges, then I added new edges to connect the new vertices, and finally I moved the new vertices to form a staircase. Propably not the most efficient way to do the job, but I wanted to do something simple that (I think) I understand.
Then I followed the tutorial and added seams to my model and made an UV unwrap to it. The unwrap looks exactly as it should in the UV editor, but when I continued to add an UV grid texture to the model like in the tutorial, the resulting texture is a complete mess.
I've tried to do the UV unwrap several times with little changes, but it doesn't affect the result. My best guess is that the UV map is correct, but for some reason it is not used in the model and I can't figure out what I'm doing wrong.
Edit. I appended the model to a clean project, but it still behaves the same. Here is link to the file.