tmux-which-key is a tmux plugin that provides a customizable popup menu for executing tmux commands and user-defined macros. The plugin architecture centers around a YAML configuration file that defines menu structure, keybindings, and macros, which is processed by a Python build script to generate tmux initialization code. At runtime, the plugin uses tmux's display-menu command to show nested menus and executes commands via command-aliases for performance. The plugin supports XDG directory compliance for immutable systems like NixOS and offers both YAML-based configuration (requiring Python) and direct tmux script editing alternatives. Key features include transient menus (that stay open for repeated commands), user macros (chained commands), and integration with TPM for plugin management.