Looks like it is not the right fix, or perhaps incomplete

When using with rounding the background is visible, so perhaps the fix should be applied on a per widget basis

I have a better idea, let's have 3 functions instead, and call the proper one

Looks good now

Looking great! Still clipped on the left when anti-aliasing is off. I find the - 0.5f on there questionable too, but that may be the center stroke strategy you mentioned. Perhaps we need to roundf() it?

https://github.com/Immediate-Mode-UI/Nuklear/blob/master/src/nuklear_vertex.c#L965

if (list->line_AA == NK_ANTI_ALIASING_ON) {
    nk_draw_list_path_rect_to(list, nk_vec2(rect.x, rect.y),
        nk_vec2(rect.x + rect.w, rect.y + rect.h), rounding);
} else {
    nk_draw_list_path_rect_to(list, nk_vec2(rect.x-0.5f, rect.y-0.5f),
        nk_vec2(rect.x + rect.w, rect.y + rect.h), rounding);
} nk_draw_list_path_fill(list,  col);