What are the "serious" libraries?

[Joe Nelson]

> > I'm looking for a good cross platform GUI library.

June Bug wrote:
> Off the top of my head I’d say SDL2 <https://www.libsdl.org> belongs
> on this list.  Really good cross-platform way to get a window open
> and put some graphics up, used in loads of games.

Sounds great for raw graphical access.

Can you also recommend a GUI library that has widgets like buttons and
text fields for more desktop-y apps?