macos: only set bg color and titlebar color if opaque

Fixes #1771

I'm not convinced this fix won't regress some other behavior, but in my
titlebar tab and native tab modes with and without transparency, this
has looked okay to me.

macos/Sources/Features/Terminal/TerminalController.swift

@@ -172,11 +172,22 @@ class TerminalController: NSWindowController, NSWindowDelegate,
         } else {
             window.titlebarFont = nil
         }
-
+        
+        // The titlebar is always updated. We don't need to worry about opacity
+        // because we handle it here.
         let backgroundColor = OSColor(ghostty.config.backgroundColor)
-        window.backgroundColor = backgroundColor
         window.titlebarColor = backgroundColor.withAlphaComponent(ghostty.config.backgroundOpacity)
-        window.updateTabBar()
+        
+        if (window.isOpaque) {
+            // Bg color is only synced if we have no transparency. This is because
+            // the transparency is handled at the surface level (window.backgroundColor
+            // ignores alpha components)
+            window.backgroundColor = backgroundColor
+            
+            // If there is transparency, calling this will make the titlebar opaque
+            // so we only call this if we are opaque.
+            window.updateTabBar()
+        }
     }
     
     /// Update all surfaces with the focus state. This ensures that libghostty has an accurate view about