As I understand it currently SVG icons are rendered using the svgSalamander library. Sadly it isn't under active development anymore and won't return to it anytime soon. I have been developing an alternative to it (https://github.com/weisJ/jsvg) and in the meantime it has become almost on par feature-wise with svgSalamander in some areas and exceeds it in some others. In particular the only missing features I am aware of are animations and element picking by position which aren't exposed through FlatSVGIcon anyway.

Note: The Jetbrains team recently switched from rendering their UI icons with Batik to JSVG, so the library seems to work well for them:
JetBrains/intellij-community@93a8fe3

I was wondering whether replacing svgSalamander with JSVG would be something to consider here? If yes I would be happy to prepare a PR :)