Make unselected tabs blend better with background color

This enables the standard effect created by a sytem tab bar, which
ensures unselected tabs blend with the window's/titlebar's background
color. This also ensures the `windowButtonsBackdrop` view matches the
color of the adjacent tab (be it selected or not).

macos/Sources/Features/Terminal/TerminalWindow.swift

@@ -27,7 +27,7 @@ class TerminalWindow: NSWindow {
         }
     }
     
-    private var windowButtonsBackdrop: NSView? = nil
+    private var windowButtonsBackdrop: WindowButtonsBackdropView? = nil
     private var windowDragHandle: WindowDragView? = nil
     private var storedTitlebarBackgroundColor: CGColor? = nil
     
@@ -51,9 +51,21 @@ class TerminalWindow: NSWindow {
 
     /// This is called by titlebarTabs changing so that we can setup the rest of our window
     private func changedTitlebarTabs(to newValue: Bool) {
-        self.titlebarAppearsTransparent = newValue
-        
         if (newValue) {
+            // 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 titlebarContainer = contentView?.superview?.subviews.first(where: {
+                $0.className == "NSTitlebarContainerView"
+            }), let effectView = titlebarContainer.descendants(withClassName: "NSVisualEffectView").first {
+                effectView.isHidden = true
+            }
+
+            self.titlebarSeparatorStyle = .none
+
             // We use the toolbar to anchor our tab bar positions in the titlebar,
             // so we make sure it's the right size/position, and exists.
             self.toolbarStyle = .unifiedCompact
@@ -180,7 +192,20 @@ class TerminalWindow: NSWindow {
             self.markHierarchyForLayout(accessoryView)
         }
     }
-    
+
+    // This is called when we open, close, switch, and reorder tabs, at which point we determine if the
+    // first tab in the tab bar is selected. If it is, we make the `windowButtonsBackdrop` color the same
+    // as that of the active tab (i.e. the titlebar's background color), otherwise we make it the same
+    // color as the background of unselected tabs.
+    override func update() {
+        super.update()
+        if let index = windowController?.window?.tabbedWindows?.firstIndex(of: self) {
+            let firstTabIsSelected = index == 0
+
+            windowButtonsBackdrop?.isHighlighted = firstTabIsSelected
+        }
+    }
+
     private func addWindowButtonsBackdrop(titlebarView: NSView, toolbarView: NSView) {
         // If we already made the view, just make sure it's unhidden and correctly placed as a subview.
         if let view = windowButtonsBackdrop {
@@ -188,32 +213,22 @@ class TerminalWindow: NSWindow {
             view.isHidden = false
             titlebarView.addSubview(view)
             view.leftAnchor.constraint(equalTo: toolbarView.leftAnchor).isActive = true
-            view.rightAnchor.constraint(equalTo: toolbarView.leftAnchor, constant: 80).isActive = true
+            view.rightAnchor.constraint(equalTo: toolbarView.leftAnchor, constant: 78).isActive = true
             view.topAnchor.constraint(equalTo: toolbarView.topAnchor).isActive = true
             view.heightAnchor.constraint(equalTo: toolbarView.heightAnchor).isActive = true
             return
         }
         
-        let view = NSView()
+        let view = WindowButtonsBackdropView(backgroundColor: storedTitlebarBackgroundColor ?? NSColor.windowBackgroundColor.cgColor)
         view.identifier = NSUserInterfaceItemIdentifier("_windowButtonsBackdrop")
         titlebarView.addSubview(view)
         
         view.translatesAutoresizingMaskIntoConstraints = false
         view.leftAnchor.constraint(equalTo: toolbarView.leftAnchor).isActive = true
-        view.rightAnchor.constraint(equalTo: toolbarView.leftAnchor, constant: 80).isActive = true
+        view.rightAnchor.constraint(equalTo: toolbarView.leftAnchor, constant: 78).isActive = true
         view.topAnchor.constraint(equalTo: toolbarView.topAnchor).isActive = true
         view.heightAnchor.constraint(equalTo: toolbarView.heightAnchor).isActive = true
-        view.wantsLayer = true
-        
-        // This is jank but this makes the background color for light themes on the button
-        // backdrop look MUCH better. I couldn't figure out a perfect color to use that works
-        // for both so we just check the appearance.
-        if effectiveAppearance.name == .aqua {
-            view.layer?.backgroundColor = CGColor(genericGrayGamma2_2Gray: 0.95, alpha: 1)
-        } else {
-            view.layer?.backgroundColor = CGColor(genericGrayGamma2_2Gray: 0.0, alpha: 0.45)
-        }
-        
+
         windowButtonsBackdrop = view
     }
     
@@ -283,3 +298,45 @@ fileprivate class WindowDragView: NSView {
         addCursorRect(bounds, cursor: .openHand)
     }
 }
+
+// A view that matches the color of selected and unselected tabs in the adjacent tab bar.
+fileprivate class WindowButtonsBackdropView: NSView {
+    private let overlayLayer = PlusDarkerBlendingModeLayer()
+    private let backgroundColor: CGColor
+    private let isLightTheme: Bool
+
+    var isHighlighted: Bool = true {
+        didSet {
+            if isLightTheme {
+                overlayLayer.isHidden = isHighlighted
+                layer?.backgroundColor = backgroundColor
+            } else {
+                overlayLayer.isHidden = true
+                layer?.backgroundColor = isHighlighted ? backgroundColor : CGColor(genericGrayGamma2_2Gray: 0.0, alpha: 0.45)
+            }
+
+        }
+    }
+
+    required init?(coder: NSCoder) {
+        fatalError("init(coder:) has not been implemented")
+    }
+
+    init(backgroundColor: CGColor) {
+        self.backgroundColor = backgroundColor
+        self.isLightTheme = NSColor(cgColor: backgroundColor)!.isLightColor
+
+        super.init(frame: .zero)
+
+
+        wantsLayer = true
+
+        overlayLayer.frame = layer!.bounds
+        overlayLayer.autoresizingMask = [.layerWidthSizable, .layerHeightSizable]
+        overlayLayer.backgroundColor = CGColor(genericGrayGamma2_2Gray: 0.95, alpha: 1)
+
+        layer?.addSublayer(overlayLayer)
+
+        isHighlighted = true
+    }
+}