macos: configurable titlebar fonts

Add support for configurable fonts for window and tab titles. This is
only implemented for macOS (and could be macOS-only if other platforms
aren't able to support this using their windowing toolkits). It plays
nicely with regular and titlebar tabs.

macos/Sources/Features/Terminal/TerminalController.swift

@@ -184,6 +184,13 @@ class TerminalController: NSWindowController, NSWindowDelegate,
             let appearance = NSAppearance(named: color.isLightColor ? .aqua : .darkAqua)
             window.appearance = appearance
         }
+
+        // Set the font for the window and tab titles.
+        if let titleFontName = ghostty.config.windowTitleFontFamily {
+            window.titlebarFont = NSFont(name: titleFontName, size: NSFont.systemFontSize)
+        } else {
+            window.titlebarFont = nil
+        }
     }
     
     /// Update all surfaces with the focus state. This ensures that libghostty has an accurate view about
@@ -261,7 +268,6 @@ class TerminalController: NSWindowController, NSWindowDelegate,
         if (ghostty.config.macosTitlebarTabs) {
             window.tabbingMode = .preferred
             window.titlebarTabs = true
-            syncAppearance()
             DispatchQueue.main.async {
                 window.tabbingMode = .automatic
             }
@@ -305,6 +311,9 @@ class TerminalController: NSWindowController, NSWindowDelegate,
                 window.tabGroup?.removeWindow(window)
             }
         }
+
+        // Apply any additional appearance-related properties to the new window.
+        syncAppearance()
     }
     
     // Shows the "+" button in the tab bar, responds to that click.

macos/Sources/Features/Terminal/TerminalToolbar.swift

@@ -16,6 +16,16 @@ class TerminalToolbar: NSToolbar, NSToolbarDelegate {
         }
     }
 
+    var titleFont: NSFont? {
+        get {
+            titleTextField.font
+        }
+
+        set {
+            titleTextField.font = newValue
+        }
+    }
+
     override init(identifier: NSToolbar.Identifier) {
         super.init(identifier: identifier)
         

macos/Sources/Features/Terminal/TerminalWindow.swift

@@ -8,6 +8,12 @@ class TerminalWindow: NSWindow {
     
     // MARK: - NSWindow
     
+    override var title: String {
+        didSet {
+            tab.attributedTitle = attributedTitle
+        }
+    }
+
     override func becomeKey() {
         // This is required because the removeTitlebarAccessoryViewControlle hook does not
         // catch the creation of a new window by "tearing off" a tab from a tabbed window.
@@ -20,6 +26,8 @@ class TerminalWindow: NSWindow {
         if titlebarTabs {
             updateNewTabButtonOpacity()
         }
+
+        tab.attributedTitle = attributedTitle
     }
 
     override func resignKey() {
@@ -28,6 +36,8 @@ class TerminalWindow: NSWindow {
         if titlebarTabs {
             updateNewTabButtonOpacity()
         }
+
+        tab.attributedTitle = attributedTitle
     }
 
     // MARK: - Titlebar Tabs
@@ -39,6 +49,38 @@ class TerminalWindow: NSWindow {
         }
     }
     
+    // Used to set the titlebar font.
+    var titlebarFont: NSFont? {
+        didSet {
+            titlebarTextField?.font = titlebarFont
+            tab.attributedTitle = attributedTitle
+
+            if let toolbar = toolbar as? TerminalToolbar {
+                toolbar.titleFont = titlebarFont
+            }
+        }
+    }
+
+    // Find the NSTextField responsible for displaying the titlebar's title.
+    private var titlebarTextField: NSTextField? {
+        guard let titlebarContainer = contentView?.superview?.subviews
+            .first(where: { $0.className == "NSTitlebarContainerView" }) else { return nil }
+        guard let titlebarView = titlebarContainer.subviews
+            .first(where: { $0.className == "NSTitlebarView" }) else { return nil }
+        return titlebarView.subviews.first(where: { $0 is NSTextField }) as? NSTextField
+    }
+
+    // Return a styled representation of our title property.
+    private var attributedTitle: NSAttributedString? {
+        guard let titlebarFont else { return nil }
+
+        let attributes: [NSAttributedString.Key: Any] = [
+            .font: titlebarFont,
+            .foregroundColor: isKeyWindow ? NSColor.labelColor : NSColor.secondaryLabelColor,
+        ]
+        return NSAttributedString(string: title, attributes: attributes)
+    }
+
     private var windowButtonsBackdrop: WindowButtonsBackdropView? = nil
     private var windowDragHandle: WindowDragView? = nil
     private var storedTitlebarBackgroundColor: CGColor? = nil

macos/Sources/Ghostty/Ghostty.Config.swift

@@ -198,6 +198,15 @@ extension Ghostty {
             return v
         }
 
+        var windowTitleFontFamily: String? {
+            guard let config = self.config else { return nil }
+            var v: UnsafePointer<Int8>? = nil
+            let key = "window-title-font-family"
+            guard ghostty_config_get(config, &v, key, UInt(key.count)) else { return nil }
+            guard let ptr = v else { return nil }
+            return String(cString: ptr)
+        }
+
         var macosTitlebarTabs: Bool {
             guard let config = self.config else { return false }
             var v = false;

src/config/Config.zig

@@ -603,6 +603,11 @@ keybind: Keybinds = .{},
 /// borders.
 @"window-decoration": bool = true,
 
+/// The font that will be used for the application's window and tab titles.
+///
+/// This is currently only supported on macOS.
+@"window-title-font-family": ?[:0]const u8 = null,
+
 /// The theme to use for the windows. Valid values:
 ///
 ///   * `auto` - Determine the theme based on the configured terminal