Before this commit, zooming in was growing node height relative to width, while zooming out was shrinking node height.

I don't quite completely understand why that was happening, or why this seems to work better. node_height before was comparing two values, node.h and 3*line_height. line_height was scaled based on Viewport.zoom. node.h was computed based on box_height which was unscaled. So far, so good. But scaling box_height doesn't fix anything.