I'm using blender 2.79 and have not modified any of its code, but for some reason I have to use the space bar and search for "Mark Seam" in order to use this function. I should not that I don't see it in blender 2.8 either. Am I missing something? I thought it was always under the Edge's Menu!
Menu: Mesh ‣ Edges ‣ Mark Seam/Clear Seam is gone!
Has anyone had this problem, or know how to fix it?