Someone may reply to me with one. I'd have to look at it and analyze. But I know I'm not going to get dozens of citations of high-quality toolkits of, say, the calibre that I can put rich text widgets with high-quality multi-font unicode support and everything else I want out of such a widget, into a tree control with those rich text widgets as my leaves, and have a million node's worth of rich text controls because they can be lazy-loaded based on visibility, just to come up with one use case.
Reminds of early-mid-00s and Delphi components. The standard components were fine, but lacked many important features (including Unicode!), so you'd hunt/shop around for third-party components.
That probably wasn't as much of a problem for a working adult but it wasn't much fun as a broke teenager itching to make things (as I was at that point).
The newer UI Toolkits have only labels. Buttons, scrollbars, borders, menus, comboboxes cannot be implemented with the current state of the art technology. /s
That's to say that a useful GUI toolkit has typically been a whole lot more than simply "draw me some widgets and let me poll on input events"
I wonder if that's really still as true though in newer (not C/C++) languages like Rust for example, where you can reuse a lot more of what the ecosystem and language already provide.