There is microui if you want another one. Much more barebones as the name implies, but very easy to extend! Pleasant even if a bit quirky. Also nuklear, but haven't really gone much in depth with it.
https://floooh.github.io/sokol-html5/sgl-microui-sapp.html
https://floooh.github.io/sokol-html5/nuklear-sapp.html
IMHO Nuklear goes a bit too much into the 'immediate mode design purity' direction which makes it a bit less convenient to use than Dear ImGui.
Microui is indeed a very minimal UI, but might be useful for just adding a small controller UI (similar to dat.gui in the JS world).
Sorry about the confusion.
E.g. in microui 1.x defining a window worked like this:
static mu_Container window;
if (mu_begin_window(ctx, &window, "Style Editor")) { ... }
if (mu_begin_window(ctx, "Style Editor", mu_rect(350, 250, 300, 240))) { ... }