"Mosaic view" should display the star button
Mosaic view is the only one without the star button. I view my YouTube feed in mosaic mode and I often need to boorkmark videos to view them later. It's a bit uncomfortable to have to expand each item for the star button to be displayed.
I don't like idea of bloating mosaic view with additional buttons. But I think it should be possible to show them when pressing Shift/Alt/Ctrl/Cmd key.