I was trying to think too much about it. So right, GUI. But there is too few true multiplatform applications anyway. You do have ports for different platforms and those ports can each use appropriate gui toolkit.

Yeah. I love it when companies put out a versions of a product that look and feel distinct and standalone on each platform.

Though IMO some apps do get away with being uniformly alien everywhere, for example Spotify. Can't put my finger on why.