feat: Select Links On Right Click If Present

[mkasberg]

This is at least a partial solution to #2107. (It's not totally clear to me whether we'd want to leave #2107 open even after this change to support additional right-click actions for OSC8 URLs...)

AI Disclosure: I used Gemini CLI to help me with this PR because while I have many years of programming experience, this is my first time writing Zig. I prototyped a couple different approaches with AI before landing on this one, so AI generated various prototypes and I chose the final imlementation. I've verified that my code compiles and works as intended.

When a user right-clicks, and there's no existing selection, the existing behavior is to try to select the word under the cursor:

ghostty/src/Surface.zig

Lines 3740 to 3742 in 3548acf

	const sel = screen.selectWord(pin) orelse break :sel;
	try self.setSelection(sel);
	try self.queueRender();

This PR proposes tweaking that behavior slightly: If there's a link under our cursor, as determined by linkAtPos, select the link. Otherwise, select the word as before.

I think this behavior provides a better user experience because it seems exceptionally rare that a user is right-clicking on a link but would prefer to take action on the word rather than the (underlined) link. The new behavior also matches other terminals like iTerm and Gnome Terminal, as shown in #2107.

It's worth noting that linkAtPos already does the right thing in terms of checking the links from config and their highlight/hover states (modified by Ctrl or Super depending on platform). (Docs)

ghostty/src/Surface.zig

Lines 3896 to 3901 in 3548acf

	// Go through each link and see if we clicked it
	for (self.config.links) |link| {
	switch (link.highlight) {
	.always, .hover => {},
	.always_mods, .hover_mods => |v| if (!v.equal(mouse_mods)) continue,
	}

It also therefore respects link-url from config.

ghostty/src/config/Config.zig

Lines 3411 to 3416 in 3548acf

	// Add our default link for URL detection
	try result.link.links.append(alloc, .{
	.regex = url.regex,
	.action = .{ .open = {} },
	.highlight = .{ .hover_mods = inputpkg.ctrlOrSuper(.{}) },
	});

By using linkAtPos, we get all that behavior for free. In practical terms, that means:

• If I'm holding Ctrl so a link is underlined and I right click on it, it selects the underlined link.
• If I'm not holding Ctrl and I right click on a link that is not underlined, it selects the word as before.
• This behavior respects per-platform key bindings and user config settings.

linkAtPos requires that the render state mutex is held. I believe it's safe to call because we're inside a block holding the mutex:

ghostty/src/Surface.zig

Lines 3702 to 3704 in 3548acf

	if (button == .right and action == .press) sel: {
	self.renderer_state.mutex.lock();
	defer self.renderer_state.mutex.unlock();

Original Behavior:
(first without ctrl, then with ctrl)

existing_behavior.mp4

New Behavior:
(first without ctrl, then with ctrl, then pasting)

new_behavior.mp4