Gnergy

oddly enough all these things have repeatedly appeared, disappeared, and reappeared in my lifetime, e.g. titlebars on Windows became transparent with Vista, then stopped being so in, I think, Windows 8?

I would start here: https://ryf.fsf.org/products?category=1&vendor=All&sort_by=created&sort_order=DESC or if that is too limiting, here: https://kde.org/hardware/

Nowhere close to the >95% market share IE had in the early 2000s.