macos: use the configured unfocused split opacity

macos/Sources/Features/Primary Window/PrimaryView.swift

@@ -96,6 +96,7 @@ struct PrimaryView: View {
             
             Ghostty.TerminalSplit(onClose: Self.closeWindow, baseConfig: self.baseConfig)
                 .ghosttyApp(ghostty.app!)
+                .ghosttyConfig(ghostty.config!)
                 .background(WindowAccessor(window: $window))
                 .onReceive(gotoTab) { onGotoTab(notification: $0) }
                 .onReceive(toggleFullscreen) { onToggleFullscreen(notification: $0) }

macos/Sources/Ghostty/AppState.swift

@@ -350,15 +350,28 @@ private struct GhosttyAppKey: EnvironmentKey {
     static let defaultValue: ghostty_app_t? = nil
 }
 
+private struct GhosttyConfigKey: EnvironmentKey {
+    static let defaultValue: ghostty_config_t? = nil
+}
+
 extension EnvironmentValues {
     var ghosttyApp: ghostty_app_t? {
         get { self[GhosttyAppKey.self] }
         set { self[GhosttyAppKey.self] = newValue }
     }
+    
+    var ghosttyConfig: ghostty_config_t? {
+        get { self[GhosttyConfigKey.self] }
+        set { self[GhosttyConfigKey.self] = newValue }
+    }
 }
 
 extension View {
     func ghosttyApp(_ app: ghostty_app_t?) -> some View {
         environment(\.ghosttyApp, app)
     }
+    
+    func ghosttyConfig(_ config: ghostty_config_t?) -> some View {
+        environment(\.ghosttyConfig, config)
+    }
 }

macos/Sources/Ghostty/SurfaceView.swift

@@ -48,10 +48,20 @@ extension Ghostty {
         // Maintain whether our window has focus (is key) or not
         @State private var windowFocus: Bool = true
         
+        @Environment(\.ghosttyConfig) private var ghostty_config
+        
         // This is true if the terminal is considered "focused". The terminal is focused if
         // it is both individually focused and the containing window is key.
         private var hasFocus: Bool { surfaceFocus && windowFocus }
         
+        // The opacity of the rectangle when unfocused.
+        private var unfocusedOpacity: Double {
+            var opacity: Double = 0.85
+            let key = "unfocused-split-opacity"
+            _ = ghostty_config_get(ghostty_config, &opacity, key, UInt(key.count))
+            return 1 - opacity
+        }
+        
         var body: some View {
             ZStack {
                 // We use a GeometryReader to get the frame bounds so that our metal surface
@@ -130,7 +140,7 @@ extension Ghostty {
                     Rectangle()
                         .fill(.white)
                         .allowsHitTesting(false)
-                        .opacity(0.15)
+                        .opacity(unfocusedOpacity)
                 }
             }
         }