`scroll_*` bindings should respect "performable" and do nothing in alt screen

[mitchellh]

Discussed in #6719

^{Originally posted by sirburpalot March 14, 2025}
Key bindings for actions like scroll_page_up, scroll_to_top, etc., when marked with the "performable:" prefix, should be ignored in fullscreen applications like nano or far2l, because such applications do not typically utilize the scrollback buffer.

How to reproduce the bug:

1. Add keybind = performable:shift+home=scroll_to_top to Ghostty's config.
2. Launch nano.
3. Type a string of random characters and press Shift+Home. The Shift+Home shortcut will not be passed to nano.

Notes

I am constraining this issue specifically to alt screen, but technically scroll should respect performable in ANY scenario where there is no scrollback, so if we fix it that way that'd be best. ❤

Marked as a feature because performable is documented as not working for everything yet.

[jcollie]

This isn't going to be as easy as I first thought, basically because scrolling is an asynchronous operation that has to send a message to the IO thread. The keybind requires a synchronous response.

ghostty/src/Surface.zig

Lines 4066 to 4070 in bd7c5cc

	.scroll_to_top => {
	self.io.queueMessage(.{
	.scroll_viewport = .{ .top = {} },
	}, .unlocked);
	},

[mitchellh]

We can grab the IO lock for this and inspect the terminal state. We could probably do this whole operation with a lock rather than a message but I’d have to double check the implementation…