Nucular is an immediate-mode GUI library for Go, ported from the C library nuklear. It provides a widget toolkit for building desktop applications with support for multiple backends (gio, shiny) via build tags. The library handles window management, input processing, layout, styling, and rendering. Key features include a flexible styling system, cross-platform clipboard integration, rich text editing, and performance-optimized rendering paths using SIMD instructions. Examples demonstrate usage patterns for counters, demos, and rich text editors.