macOS: Add new config to show proxy icon only on hover

[mitchellh]

Discussed in #5819

^{Originally posted by edrozenberg February 16, 2025}
With recent MacOS releases, in apps such as Finder, Preview and others, the titlebar proxy icon is normally hidden until mouseover.

Currently Ghostty does the following via macos-titlebar-proxy-icon:

• visible - Always show the proxy icon, proxy icon functionality enabled
• hidden - Hide the proxy icon & disable proxy icon functionality
  (https://ghostty.org/docs/config/reference#macos-titlebar-proxy-icon)

There is currently no way to match standard MacOS behavior because with hidden there is no proxy icon at all.

Suggestion:
Separate icon visibility setting from functionality enabled setting -
macos-titlebar-proxy-icon = < visible | hidden | false/disabled >

• visible - Always show the proxy icon (older MacOS behavior)
• hidden - Hide the proxy icon until mouseover (recent MacOS behavior)
• false/disabled - No proxy icon at all, disable proxy icon functionality

Mac Finder w/ proxy icon showing on mouseover:
https://github.com/user-attachments/assets/3d34d4a7-8a03-4138-8b24-e9d26871b5fa

Assumption:
With proxy icon false/disabled, proxy icon functionality is also disabled (dragging from the titlebar title, right-clicking titlebar title to see dir paths). This would match the behavior of other apps that don't use proxy icons (for ex. Apple Pages, Numbers, Keynote) which have no proxy icon and no proxy icon-like behavior in the titlebar.

Motivation:

• Match appearance/behavior of recent MacOS versions where most apps that use proxy icons have them hidden until mouseover
• Be able to use proxy icon functionality while having the icon hidden unless mouseover'd, for a cleaner look
• Also maintain the option to fully disable proxy icon functionality (icon & actions both disabled)

Proposal

1. Accept the above discussion proposal as given.
2. Make the default hidden. This breaks from Terminal.app but I agree with OP that this is the future macOS design appears to be heading and Terminal.app is simply... old.