refactor: derive gutter background color from text view appearance

The gutter background color now updates whenever the text view appearance
changes. This ensures the gutter always matches the text view as the system
appearance changes between light and dark mode.

Also moved a few appearance-related updates into the appearance change
callback to keep them together.

Author: krzyzanowskim

Sources/STTextViewAppKit/Gutter/STGutterView.swift

@@ -60,12 +60,12 @@ open class STGutterView: NSView, NSDraggingSource {
     @Invalidating(.display)
     open var highlightSelectedLine: Bool = false
 
-    /// A Boolean value that indicates whether the receiver draws its background. Default true.
     @Invalidating(.display, .background)
-    open var drawsBackground: Bool = true
-
-    @Invalidating(.display)
-    internal var backgroundColor: NSColor = NSColor.controlBackgroundColor
+    internal var backgroundColor: NSColor = NSColor.controlBackgroundColor {
+        didSet {
+            layer?.backgroundColor = backgroundColor.cgColor
+        }
+    }
 
     /// The background color of the highlighted line.
     @Invalidating(.display)
@@ -126,11 +126,7 @@ open class STGutterView: NSView, NSDraggingSource {
     }
 
     fileprivate func updateBackgroundColor() {
-        if drawsBackground {
-            layer?.backgroundColor = backgroundColor.cgColor
-        } else {
-            layer?.backgroundColor = nil
-        }
+        self.backgroundColor = backgroundColor
     }
 
     public func addMarker(_ marker: STGutterMarker) {

Sources/STTextViewAppKit/STTextView.swift

@@ -717,11 +717,11 @@ import AVFoundation
         effectiveAppearance.performAsCurrentDrawingAppearance { [weak self] in
             guard let self else { return }
             self.backgroundColor = self.backgroundColor
-        }
 
-        self.updateSelectedRangeHighlight()
-        self.layoutGutter()
-        self.updateSelectedLineHighlight()
+            self.updateSelectedRangeHighlight()
+            self.layoutGutter()
+            self.updateSelectedLineHighlight()
+        }
     }
 
     open override func viewDidMoveToSuperview() {