Fix WKWebView scrollIndicatorStyle workaround on iOS 15.4

Reader/AppDelegate.swift

@@ -115,21 +115,28 @@ class AppDelegate: UIResponder, UIApplicationDelegate {
         }
     }
     
+    // this workaround is no longer necessary (in fact, it causes a stack overflow) post-iOS 15.4 because of:
+    // https://github.com/WebKit/WebKit/commit/1dbd34cf01d8b5aedcb8820b13cb6553ed60e8ed
+    @available(iOS, obsoleted: 15.4)
     private func swizzleWKWebView() {
+        if #available(iOS 15.4, *) {
+        } else {
             let selector = Selector(("_updateScrollViewBackground"))
             var originalIMP: IMP?
             let imp = imp_implementationWithBlock({ (self: WKWebView) in
                 if let originalIMP = originalIMP {
                     let original = unsafeBitCast(originalIMP, to: (@convention(c) (WKWebView, Selector) -> Void).self)
                     original(self, selector)
-            } else {
-                os_log(.error, "Missing originalIMP for -[WKWebView _updateScrollViewBackground], did WebKit change?")
                 }
                 
                 self.scrollView.indicatorStyle = .default
                 
             } as (@convention(block) (WKWebView) -> Void))
             originalIMP = class_replaceMethod(WKWebView.self, selector, imp, "v@:")
+            if originalIMP == nil {
+                os_log(.error, "Missing originalIMP for -[WKWebView _updateScrollViewBackground], did WebKit change?")
+            }
+        }
     }
     
     @objc private func showPreferences() {

Reader/Screens/Read/ReadViewController.swift

@@ -22,6 +22,8 @@ class ReadViewController: UIViewController {
     let fervorController: FervorController
     let item: Item
     
+    private var webView: WKWebView!
+    
     #if targetEnvironment(macCatalyst)
     private var itemReadObservation: NSKeyValueObservation?
     #endif
@@ -53,7 +55,7 @@ class ReadViewController: UIViewController {
         view.backgroundColor = .appBackground
         view.addInteraction(StretchyMenuInteraction(delegate: self))
         
-        let webView = WKWebView()
+        webView = WKWebView()
         webView.translatesAutoresizingMaskIntoConstraints = false
         webView.navigationDelegate = self
         webView.uiDelegate = self
@@ -83,6 +85,36 @@ class ReadViewController: UIViewController {
         #endif
     }
     
+    override func viewWillAppear(_ animated: Bool) {
+        super.viewWillAppear(animated)
+        
+        updateScrollIndicatorStyle()
+    }
+    
+    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
+        super.traitCollectionDidChange(previousTraitCollection)
+        
+        updateScrollIndicatorStyle()
+    }
+    
+    private func updateScrollIndicatorStyle() {
+        guard #available(iOS 15.4, *) else {
+            // different workaround pre-iOS 15.4
+            return
+        }
+        // can't use .default because that causes the WKScrollView to think the indicator style has not been set by the client (us):
+        // https://github.com/WebKit/WebKit/blob/1dbd34cf01d8b5aedcb8820b13cb6553ed60e8ed/Source/WebKit/UIProcess/ios/WKScrollView.mm#L247
+        // if that happens, it goes back to trying to set it based on background color which doesn't work because we give it a clear background
+        // https://github.com/WebKit/WebKit/blob/d085008d57f784d0913a8c37351f60b4a0eb8a3a/Source/WebKit/UIProcess/API/ios/WKWebViewIOS.mm#L562
+        // https://github.com/WebKit/WebKit/blob/d085008d57f784d0913a8c37351f60b4a0eb8a3a/Source/WebKit/UIProcess/API/ios/WKWebViewIOS.mm#L520
+        // so, we set it ourselves based on the user interface style
+        if traitCollection.userInterfaceStyle == .dark {
+            webView.scrollView.indicatorStyle = .white
+        } else {
+            webView.scrollView.indicatorStyle = .black
+        }
+    }
+    
     private static let css = try! String(contentsOf: Bundle.main.url(forResource: "read", withExtension: "css")!)
     private static let js = try! String(contentsOf: Bundle.main.url(forResource: "read", withExtension: "js")!)