Add toggle_background_opacity keybind action

[mitchellh]

Discussed in #4664

^{Originally posted by yanskun January 5, 2025}
I like to use the terminal with a transparent background.
However, when sharing a screen for pair programming, etc., it seems that noise can be introduced depending on the other person, so I would like to be able to easily turn off transparency.

iTerm has a Toggle Transparency feature that allows you to switch between custom opacity and non-transparent by pressing cmd + u.
https://stackoverflow.com/questions/34159559/toggle-opacity-with-hotkey-in-terminal-iterm#:~:text=cmd%20%2B%20u%20toggles%20the%20transparency.

Does Ghostty have it?

Since Alacritty doesn't seem to have this feature, I prepared a shell script to change the config directly.

If Ghostty doesn't have this feature, I'll prepare a Shell Script.
However, if Ghostty has it, I'd like to use it.
(If I get excited, I'd like to implement it directly in Ghostty and submit a PR.)

Proposal

Add a new keybind action toggle_background_opacity. This does nothing if background-opacity is 1+.

If background-opacity is less than 1, then this will either set the window transparent or not depending on its current state.

When reloading the config, it is acceptable that background opacity is reapplied. This will make implementing this easier.

This should be implemented for macOS, X11, and Wayland.