New feature: Immersion Mode

App/Navigation/NavigationController.swift

@@ -228,6 +228,27 @@ final class NavigationController: UINavigationController, Themeable {
     var isScrolledFromTop = false
     private var lastAppliedScrollProgress: CGFloat = -1
 
+    // MARK: Scroll-driven appearance caches
+    //
+    // `updateNavigationBarTintForScrollProgress` rebuilds a UINavigationBarAppearance on
+    // every scroll delta above 0.005. Cache the expensive pieces so scroll-driven updates
+    // don't re-allocate/redraw identical resources each frame.
+    private var cachedGradientImage: UIImage?
+    private var cachedGradientImageColor: UIColor?
+    private lazy var cachedBackIndicatorTemplate: UIImage? = UIImage(named: "back")?.withRenderingMode(.alwaysTemplate)
+    private lazy var cachedBackIndicatorLabelTinted: UIImage? = UIImage(named: "back")?.withTintColor(.label, renderingMode: .alwaysOriginal)
+
+    @available(iOS 26.0, *)
+    private func gradientBackgroundImage(from color: UIColor) -> UIImage? {
+        if let cached = cachedGradientImage, cachedGradientImageColor == color {
+            return cached
+        }
+        let image = createGradientBackgroundImage(from: color)
+        cachedGradientImage = image
+        cachedGradientImageColor = color
+        return image
+    }
+
     func statusBarEnterLightBackground() {
         isDarkContentBackground = false
         setNeedsStatusBarAppearanceUpdate()
@@ -321,6 +342,8 @@ final class NavigationController: UINavigationController, Themeable {
 
     func themeDidChange() {
         lastAppliedScrollProgress = -1
+        cachedGradientImage = nil
+        cachedGradientImageColor = nil
         updateNavigationBarAppearance(with: theme)
 
         if #available(iOS 26.0, *) {
@@ -572,13 +595,26 @@ final class NavigationController: UINavigationController, Themeable {
 
         let progressValue = CGFloat(progress.floatValue)
 
+        // Snap to the extremes: near-0 and near-1 scrolls render identically to 0 and 1,
+        // so collapse them into a single stable value. Without this, oscillations around
+        // the boundaries (e.g. 0.003 → 0.009 → 0.002) each clear the 0.005 delta gate
+        // and rebuild the appearance even though the visual result is unchanged.
+        let snappedProgress: CGFloat
+        if progressValue < ScrollProgress.atTop {
+            snappedProgress = 0
+        } else if progressValue > ScrollProgress.fullyScrolled {
+            snappedProgress = 1
+        } else {
+            snappedProgress = progressValue
+        }
+
         // Avoid redundant appearance rebuilds when progress hasn't changed.
-        if abs(progressValue - lastAppliedScrollProgress) < 0.005 {
+        if abs(snappedProgress - lastAppliedScrollProgress) < 0.005 {
             return
         }
-        lastAppliedScrollProgress = progressValue
+        lastAppliedScrollProgress = snappedProgress
 
-        updateNavigationBarBackgroundWithProgress(progressValue)
+        updateNavigationBarBackgroundWithProgress(snappedProgress)
 
         if progressValue < ScrollProgress.atTop {
             isScrolledFromTop = false
@@ -649,7 +685,7 @@ final class NavigationController: UINavigationController, Themeable {
                 return
             }
 
-            if let gradientImage = createGradientBackgroundImage(from: gradientBaseColor) {
+            if let gradientImage = gradientBackgroundImage(from: gradientBaseColor) {
                 appearance.backgroundImage = gradientImage
                 let overlayAlpha = 1.0 - progress
                 appearance.backgroundColor = opaqueColor.withAlphaComponent(overlayAlpha)
@@ -667,14 +703,11 @@ final class NavigationController: UINavigationController, Themeable {
 
     @available(iOS 26.0, *)
     private func configureBackIndicator(for appearance: UINavigationBarAppearance, progress: CGFloat) {
-        if progress > ScrollProgress.fullyScrolled {
-            if let backImage = UIImage(named: "back")?.withTintColor(.label, renderingMode: .alwaysOriginal) {
-                appearance.setBackIndicatorImage(backImage, transitionMaskImage: backImage)
-            }
-        } else {
-            if let backImage = UIImage(named: "back")?.withRenderingMode(.alwaysTemplate) {
-                appearance.setBackIndicatorImage(backImage, transitionMaskImage: backImage)
-            }
+        let backImage = progress > ScrollProgress.fullyScrolled
+            ? cachedBackIndicatorLabelTinted
+            : cachedBackIndicatorTemplate
+        if let backImage {
+            appearance.setBackIndicatorImage(backImage, transitionMaskImage: backImage)
         }
     }
 

App/View Controllers/Posts/PostsPageSettingsViewController.swift

@@ -16,6 +16,7 @@ final class PostsPageSettingsViewController: ViewController, UIPopoverPresentati
     @FoilDefaultStorage(Settings.darkMode) private var darkMode
     @FoilDefaultStorage(Settings.enableHaptics) private var enableHaptics
     @FoilDefaultStorage(Settings.fontScale) private var fontScale
+    @FoilDefaultStorage(Settings.immersiveModeEnabled) private var immersiveModeEnabled
     @FoilDefaultStorage(Settings.showAvatars) private var showAvatars
     @FoilDefaultStorage(Settings.loadImages) private var showImages
 
@@ -42,7 +43,7 @@ final class PostsPageSettingsViewController: ViewController, UIPopoverPresentati
         performHapticFeedback()
         showImages = sender.isOn
     }
-    
+
     @IBOutlet private var scaleTextLabel: UILabel!
     @IBOutlet private var scaleTextStepper: UIStepper!
     @IBAction private func scaleStepperDidChange(_ sender: UIStepper) {
@@ -64,8 +65,13 @@ final class PostsPageSettingsViewController: ViewController, UIPopoverPresentati
         darkMode = sender.isOn
     }
 
+    private var immersiveModeStack: UIStackView?
+    private var immersiveModeLabel: UILabel?
+    private var immersiveModeSwitch: UISwitch?
+
     @objc private func toggleImmersiveMode(_ sender: UISwitch) {
         performHapticFeedback()
+        immersiveModeEnabled = sender.isOn
     }
 
     // MARK: - Helper Methods
@@ -132,6 +138,60 @@ final class PostsPageSettingsViewController: ViewController, UIPopoverPresentati
             .receive(on: RunLoop.main)
             .sink { [weak self] in self?.imagesSwitch?.isOn = $0 }
             .store(in: &cancellables)
+
+        $immersiveModeEnabled
+            .receive(on: RunLoop.main)
+            .sink { [weak self] in self?.immersiveModeSwitch?.isOn = $0 }
+            .store(in: &cancellables)
+
+        DispatchQueue.main.async { [weak self] in
+            self?.setupImmersiveModeUI()
+        }
+    }
+
+    override func viewDidLayoutSubviews() {
+        super.viewDidLayoutSubviews()
+        updatePreferredContentSize()
+    }
+
+    private func setupImmersiveModeUI() {
+        guard isViewLoaded, immersiveModeStack == nil else { return }
+
+        let label = UILabel()
+        label.text = "Immersive Mode"
+        label.font = UIFont.preferredFont(forTextStyle: .body)
+        label.textColor = theme["sheetTextColor"] ?? UIColor.label
+        immersiveModeLabel = label
+
+        let modeSwitch = UISwitch()
+        modeSwitch.isOn = immersiveModeEnabled
+        modeSwitch.onTintColor = theme["settingsSwitchColor"]
+        modeSwitch.addTarget(self, action: #selector(toggleImmersiveMode(_:)), for: .valueChanged)
+        immersiveModeSwitch = modeSwitch
+
+        let stack = UIStackView(arrangedSubviews: [label, modeSwitch])
+        stack.axis = .horizontal
+        stack.distribution = .equalSpacing
+        stack.alignment = .center
+        stack.translatesAutoresizingMaskIntoConstraints = false
+        immersiveModeStack = stack
+
+        if let darkModeStack = darkModeStack,
+           let parentStack = darkModeStack.superview as? UIStackView {
+            if let index = parentStack.arrangedSubviews.firstIndex(of: darkModeStack) {
+                parentStack.insertArrangedSubview(stack, at: index + 1)
+            } else {
+                parentStack.addArrangedSubview(stack)
+            }
+        } else {
+            view.addSubview(stack)
+            NSLayoutConstraint.activate([
+                stack.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 20),
+                stack.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -20),
+                stack.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: -20),
+                stack.heightAnchor.constraint(equalToConstant: 44)
+            ])
+        }
     }
 
     private func updatePreferredContentSize() {
@@ -153,6 +213,9 @@ final class PostsPageSettingsViewController: ViewController, UIPopoverPresentati
         for uiswitch in switches {
             uiswitch.onTintColor = theme["settingsSwitchColor"]
         }
+
+        immersiveModeLabel?.textColor = theme["sheetTextColor"] ?? UIColor.label
+        immersiveModeSwitch?.onTintColor = theme["settingsSwitchColor"]
     }
 
     // MARK: UIAdaptivePresentationControllerDelegate

App/View Controllers/Posts/PostsPageTopBarLiquidGlass.swift

@@ -75,7 +75,7 @@ final class PostsPageTopBarLiquidGlass: UIView, PostsPageTopBarProtocol {
         private let label: UILabel
 
         override init(frame: CGRect) {
-            let glassEffect = UIGlassEffect()
+            let glassEffect = UIGlassEffect(style: .clear)
             glassView = UIVisualEffectView(effect: glassEffect)
             glassView.translatesAutoresizingMaskIntoConstraints = false
             glassView.isUserInteractionEnabled = false