Zenity is a bit too limited for my use cases. What are the alternatives?
It must be inside package sources, and it must be useable from within Bash scripts.
Yad (yet another dialog) is a fork of Zenity with many improvements, such as custom buttons, additional dialogs, pop-up menu in notification icon and more.
You can install yad using the following PPA,
See example demonstrations how to use yad in shell scripts.