I expect the layout would also be simpler in another way: Give every "child" div a static indentation, nothing more. They'll nest and the indentation will stack the deeper the comments go.

Right now there's already nested tables-within-tables, just to work around how table columns work in order to handle indentation (and each row's indentation is handled separately with a stretched 1x1 image with a width attribute that's calculated server-side).