This arose from, and blocks - and is now incorporated into - !167 (comment 223789) for GTK+ 4. It's an issue since the -GtkTreeView-expander-size style property was removed and replaced with just a macro hard-coded in the C file.
Rather than hard-coding this, get it from CSS. That way, themes will be able to set larger expanders than a fixed 16 px, e.g. for accessibility.