macos: fix transparent tabs on sequoia

macos/Sources/Features/Terminal/Window Styles/TitlebarTabsVenturaTerminalWindow.swift

@@ -99,9 +99,6 @@ class TitlebarTabsVenturaTerminalWindow: TerminalWindow {
             }
         }
 
-        // The remainder of this function only applies to styled tabs.
-        guard hasStyledTabs else { return }
-
 		titlebarSeparatorStyle = tabbedWindows != nil && !titlebarTabs ? .line : .none
         if titlebarTabs {
             hideToolbarOverflowButton()
@@ -170,19 +167,6 @@ class TitlebarTabsVenturaTerminalWindow: TerminalWindow {
 
     // MARK: Tab Bar Styling
 
-    // This is true if we should apply styles to the titlebar or tab bar.
-    var hasStyledTabs: Bool {
-        // If we have titlebar tabs then we always style.
-        guard !titlebarTabs else { return true }
-
-        // We style the tabs if they're transparent
-        return transparentTabs
-    }
-
-    // Set to true if the background color should bleed through the titlebar/tab bar.
-    // This only applies to non-titlebar tabs.
-    var transparentTabs: Bool = false
-
     var hasVeryDarkBackground: Bool {
         backgroundColor.luminance < 0.05
     }

macos/Sources/Features/Terminal/Window Styles/TransparentTitlebarTerminalWindow.swift

@@ -11,6 +11,13 @@ class TransparentTitlebarTerminalWindow: TerminalWindow {
     /// KVO observation for tab group window changes.
     private var tabGroupWindowsObservation: NSKeyValueObservation?
     private var tabBarVisibleObservation: NSKeyValueObservation?
+    
+    deinit {
+        tabGroupWindowsObservation?.invalidate()
+        tabBarVisibleObservation?.invalidate()
+    }
+    
+    // MARK: NSWindow
 
     override func awakeFromNib() {
         super.awakeFromNib()
@@ -19,11 +26,6 @@ class TransparentTitlebarTerminalWindow: TerminalWindow {
         // to learn why we need KVO.
         setupKVO()
     }
-    
-    deinit {
-        tabGroupWindowsObservation?.invalidate()
-        tabBarVisibleObservation?.invalidate()
-    }
 
     override func becomeMain() {
         super.becomeMain()
@@ -40,6 +42,16 @@ class TransparentTitlebarTerminalWindow: TerminalWindow {
             }
         }
     }
+    
+    override func update() {
+        super.update()
+        
+        // On macOS 13 to 15, we need to hide the NSVisualEffectView in order to allow our
+        // titlebar to be truly transparent.
+        if #unavailable(macOS 26.0) {
+            hideEffectView()
+        }
+    }
 
     // MARK: Appearance
 
@@ -86,6 +98,7 @@ class TransparentTitlebarTerminalWindow: TerminalWindow {
         guard let titlebarContainer else { return }
         titlebarContainer.wantsLayer = true
         titlebarContainer.layer?.backgroundColor = preferredBackgroundColor?.cgColor
+        effectViewIsHidden = false
     }
 
     // MARK: View Finders
@@ -149,4 +162,28 @@ class TransparentTitlebarTerminalWindow: TerminalWindow {
             self.syncAppearance(lastSurfaceConfig)
         }
     }
+    
+    // MARK: macOS 13 to 15
+    
+    // We only need to set this once, but need to do it after the window has been created in order
+    // to determine if the theme is using a very dark background, in which case we don't want to
+    // remove the effect view if the default tab bar is being used since the effect created in
+    // `updateTabsForVeryDarkBackgrounds` creates a confusing visual design.
+    private var effectViewIsHidden = false
+    
+    private func hideEffectView() {
+        guard !effectViewIsHidden else { return }
+        
+        // By hiding the visual effect view, we allow the window's (or titlebar's in this case)
+        // background color to show through. If we were to set `titlebarAppearsTransparent` to true
+        // the selected tab would look fine, but the unselected ones and new tab button backgrounds
+        // would be an opaque color. When the titlebar isn't transparent, however, the system applies
+        // a compositing effect to the unselected tab backgrounds, which makes them blend with the
+        // titlebar's/window's background.
+        if let effectView = titlebarContainer?.descendants(withClassName: "NSVisualEffectView").first {
+            effectView.isHidden = true
+        }
+
+        effectViewIsHidden = true
+    }
 }