[kovidgoyal]

The problem with using the window/viewport height is that it can change and any change means the cached thumbnails will all be invalidated, which is a huge cost. Also semantically, I dont think the books should become smaller as u shrink the window, the spines become illegible in that case.

And of course if you dont like 1/3rd screen height per shelf, you can adjust it to whatever fixed value you like in preferences.