Use emphasized/unemphasized selection colors and react to window key changes

- Add static emphasized/unemphasized colors and set background based on window
  key status in viewDidMoveToWindow
- Observe NSWindow didBecomeKey/didResignKey to update selection highlight
- Preserve appearance-change refresh
- Update hierarchy comments to STSelectionView and STSelectionHighlightView

Author: krzyzanowskim

Sources/STTextViewAppKit/Overlays/STSelectionHighlightView.swift

@@ -5,7 +5,10 @@ import AppKit
 
 final class STSelectionHighlightView: NSView {
 
-    var backgroundColor: NSColor? = .selectedTextBackgroundColor {
+    private static let emphasizedColor = NSColor.selectedTextBackgroundColor
+    private static let unemphasizedColor = NSColor.unemphasizedSelectedTextBackgroundColor
+
+    var backgroundColor: NSColor? {
         didSet {
             layer?.backgroundColor = backgroundColor?.cgColor
         }
@@ -38,4 +41,18 @@ final class STSelectionHighlightView: NSView {
             self.backgroundColor = self.backgroundColor
         }
     }
+
+    override func viewDidMoveToWindow() {
+        super.viewDidMoveToWindow()
+
+        self.backgroundColor = window?.isKeyWindow == true ? Self.emphasizedColor : Self.unemphasizedColor
+
+        NotificationCenter.default.addObserver(forName: NSWindow.didBecomeKeyNotification, object: window, queue: .main) { [weak self] notification in
+            self?.backgroundColor = Self.emphasizedColor
+        }
+
+        NotificationCenter.default.addObserver(forName: NSWindow.didResignKeyNotification, object: window, queue: .main) { [weak self] notification in
+            self?.backgroundColor = Self.unemphasizedColor
+        }
+    }
 }

Sources/STTextViewAppKit/STTextView.swift

@@ -7,8 +7,8 @@
 //          |---gutterView
 //          |---contentView
 //              |---STInsertionPointView
-//              |---selectionView
-//                  |---(STLineHighlightView | SelectionHighlightView)
+//              |---selectionView (STSelectionView)
+//                  |---(STLineHighlightView | STSelectionHighlightView)
 //              |---contentViewportView
 //                  |---STTextLayoutFragmentView
 //