background-blur-radius for macOS

include/ghostty.h

@@ -291,6 +291,10 @@ void ghostty_surface_request_close(ghostty_surface_t);
 void ghostty_surface_split(ghostty_surface_t, ghostty_split_direction_e);
 void ghostty_surface_split_focus(ghostty_surface_t, ghostty_split_focus_direction_e);
 
+// APIs I'd like to get rid of eventually but are still needed for now.
+// Don't use these unless you know what you're doing.
+void ghostty_set_window_background_blur(ghostty_surface_t, void *);
+
 #ifdef __cplusplus
 }
 #endif

macos/Sources/Ghostty/SurfaceView.swift

@@ -201,9 +201,14 @@ extension Ghostty {
             guard let window = self.window else { return }
             guard let surface = self.surface else { return }
             guard ghostty_surface_transparent(surface) else { return }
+            
+            // Set the window transparency settings
             window.isOpaque = false
             window.hasShadow = false
             window.backgroundColor = .clear
+            
+            // If we have a blur, set the blur
+            ghostty_set_window_background_blur(surface, Unmanaged.passUnretained(window).toOpaque())
         }
         
         override func resignFirstResponder() -> Bool {

src/apprt/embedded.zig

@@ -570,4 +570,34 @@ pub const CAPI = struct {
     export fn ghostty_surface_split_focus(ptr: *Surface, direction: input.SplitFocusDirection) void {
         ptr.gotoSplit(direction);
     }
+
+    /// Sets the window background blur on macOS to the desired value.
+    /// I do this in Zig as an extern function because I don't know how to
+    /// call these functions in Swift.
+    ///
+    /// This uses an undocumented, non-public API because this is what
+    /// every terminal appears to use, including Terminal.app.
+    export fn ghostty_set_window_background_blur(
+        ptr: *Surface,
+        window: *anyopaque,
+    ) void {
+        const config = ptr.app.config;
+
+        // Do nothing if we don't have background transparency enabled
+        if (config.@"background-opacity" >= 1.0) return;
+
+        // Do nothing if our blur value is zero
+        if (config.@"background-blur-radius" == 0) return;
+
+        const nswindow = objc.Object.fromId(window);
+        _ = CGSSetWindowBackgroundBlurRadius(
+            CGSDefaultConnectionForThread(),
+            nswindow.msgSend(usize, objc.sel("windowNumber"), .{}),
+            @intCast(config.@"background-blur-radius"),
+        );
+    }
+
+    /// See ghostty_set_window_background_blur
+    extern "c" fn CGSSetWindowBackgroundBlurRadius(*anyopaque, usize, c_int) i32;
+    extern "c" fn CGSDefaultConnectionForThread() *anyopaque;
 };

src/config.zig

@@ -72,6 +72,15 @@ pub const Config = struct {
     /// affect new windows, tabs, and splits.
     @"background-opacity": f64 = 1.0,
 
+    /// A positive value enables blurring of the background when
+    /// background-opacity is less than 1. The value is the blur radius to
+    /// apply. A value of 20 is reasonable for a good looking blur.
+    /// Higher values will cause strange rendering issues as well as
+    /// performance issues.
+    ///
+    /// This is only supported on macOS.
+    @"background-blur-radius": u8 = 0,
+
     /// The command to run, usually a shell. If this is not an absolute path,
     /// it'll be looked up in the PATH. If this is not set, a default will
     /// be looked up from your system. The rules for the default lookup are: