apprt: add C API for detecting background transparency per surface

include/ghostty.h

@@ -276,6 +276,7 @@ void *ghostty_app_userdata(ghostty_app_t);
 ghostty_surface_t ghostty_surface_new(ghostty_app_t, ghostty_surface_config_s*);
 void ghostty_surface_free(ghostty_surface_t);
 ghostty_app_t ghostty_surface_app(ghostty_surface_t);
+bool ghostty_surface_transparent(ghostty_surface_t);
 void ghostty_surface_refresh(ghostty_surface_t);
 void ghostty_surface_set_content_scale(ghostty_surface_t, double, double);
 void ghostty_surface_set_focus(ghostty_surface_t, bool);

macos/Sources/Ghostty/SurfaceView.swift

@@ -197,6 +197,15 @@ extension Ghostty {
             ghostty_surface_set_size(surface, UInt32(scaledSize.width), UInt32(scaledSize.height))
         }
         
+        override func viewDidMoveToWindow() {
+            guard let window = self.window else { return }
+            guard let surface = self.surface else { return }
+            guard ghostty_surface_transparent(surface) else { return }
+            window.isOpaque = false
+            window.hasShadow = false
+            window.backgroundColor = .clear
+        }
+        
         override func resignFirstResponder() -> Bool {
             let result = super.resignFirstResponder()
             

src/apprt/embedded.zig

@@ -470,6 +470,11 @@ pub const CAPI = struct {
         return surface.app;
     }
 
+    /// Returns ture if the surface has transparency set.
+    export fn ghostty_surface_transparent(surface: *Surface) bool {
+        return surface.app.config.@"background-opacity" < 1.0;
+    }
+
     /// Tell the surface that it needs to schedule a render
     export fn ghostty_surface_refresh(surface: *Surface) void {
         surface.refresh();