Widgets are bloat.

Is there a similar library but without widgets? Something really, really simple. I just want a canvas to draw, and a handler to receive mouse and keyboard events. No windows, no text, just a framebuffer.

Back in the day I was really happy with glut and opengl. You just took the three-line hello world and started drawing stuff without fuss. ImGui seems too unnecessarily complicated for me.

Shameless plug, but check out the Sokol libraries (there's also a header with a Dear ImGui rendering backend sitting on top of sokol_app.h and sokol_gfx.h):

https://github.com/floooh/sokol

...and here's a minimal standalone starter project for writing Dear ImGui apps in C I just created a couple of days ago:

https://github.com/floooh/cimgui-sokol-starterkit