feat(desktop): dark Windows title bar (DWM immersive dark mode)

composeApp/src/desktopMain/kotlin/com/nuvio/app/DesktopApp.kt

@@ -31,6 +31,7 @@ import com.nuvio.app.desktop.DesktopRuntimeLog
 import com.nuvio.app.desktop.DesktopSingleInstanceManager
 import com.nuvio.app.desktop.DesktopUriHandler
 import com.nuvio.app.desktop.DesktopWindowStateStore
+import com.nuvio.app.desktop.WindowsChromePolish
 import com.nuvio.app.desktop.WindowsNativeBootstrap
 import com.nuvio.app.desktop.WindowsUrlProtocolRegistrar
 import com.nuvio.app.features.notifications.WindowsToastHelper
@@ -56,6 +57,14 @@ private fun configureMacOsNativeAppearance() {
     System.setProperty("apple.awt.application.appearance", "NSAppearanceNameDarkAqua")
 }
 
+/**
+ * Dark native-caption styling is Windows-only (DWM immersive dark mode); macOS/Linux keep their
+ * native title bars unchanged. The window stays decorated on every platform, so native fullscreen,
+ * Aero Snap, resize and maximize are untouched.
+ */
+private val isWindowsOs: Boolean
+    get() = System.getProperty("os.name")?.contains("Windows", ignoreCase = true) == true
+
 private fun computeStartupWindowSize(): DpSize {
     val displayBounds = GraphicsEnvironment.getLocalGraphicsEnvironment()
         .defaultScreenDevice
@@ -259,6 +268,11 @@ fun main(args: Array<String>) {
                 window.contentPane.background = DesktopWindowBackground
                 window.rootPane.background = DesktopWindowBackground
                 devStreamMode?.startDiagnostics { desktopMainWindow }
+                // Recolor the native caption to dark (keeps the window decorated so native
+                // fullscreen / Aero Snap / resize all keep working).
+                if (isWindowsOs) {
+                    WindowsChromePolish.apply(window)
+                }
                 onDispose { desktopMainWindow = null }
             }
 

composeApp/src/desktopMain/kotlin/com/nuvio/app/desktop/DesktopWindowChrome.kt

@@ -0,0 +1,57 @@
+package com.nuvio.app.desktop
+
+import androidx.compose.ui.awt.ComposeWindow
+import com.sun.jna.Native
+import com.sun.jna.Pointer
+import com.sun.jna.ptr.IntByReference
+import com.sun.jna.win32.StdCallLibrary
+
+/**
+ * Darkens the **native** Windows title bar via DWM (immersive dark mode) so it matches Nuvio's
+ * near-black UI instead of the default white caption.
+ *
+ * Deliberately minimal: it sets ONLY the dark-mode caption attribute and keeps the window
+ * *decorated*. It does NOT set rounded-corner or border-color attributes — those persist into the
+ * native borderless-fullscreen window state and black out fullscreen video (DWM can no longer do
+ * direct fullscreen presentation). Window shape is left to the OS default (Win11 rounds decorated
+ * windows natively). Dark mode is a non-client/caption attribute, so it can't affect the video
+ * surface. Aero Snap / resize / maximize all keep working because the native frame is untouched.
+ *
+ * Best-effort: guarded and HRESULT-logged; no-ops on non-Windows / older OS.
+ */
+object WindowsChromePolish {
+    private const val S_OK = 0
+    // Dark caption attribute: index 20 on Win10 2004+/Win11, 19 on older Win10 (1809–1909).
+    private const val DWMWA_USE_IMMERSIVE_DARK_MODE = 20
+    private const val DWMWA_USE_IMMERSIVE_DARK_MODE_PRE_20H1 = 19
+
+    private interface Dwmapi : StdCallLibrary {
+        fun DwmSetWindowAttribute(
+            hwnd: Pointer,
+            dwAttribute: Int,
+            pvAttribute: IntByReference,
+            cbAttribute: Int,
+        ): Int
+    }
+
+    fun apply(window: ComposeWindow) {
+        if (System.getProperty("os.name")?.contains("Windows", ignoreCase = true) != true) return
+        runCatching {
+            val hwnd = Native.getWindowPointer(window) ?: run {
+                DesktopRuntimeLog.warn("window chrome polish skipped: null HWND")
+                return
+            }
+            val dwm = Native.load("dwmapi", Dwmapi::class.java)
+
+            // ONLY the dark-caption hint — NOT corner-preference or border-color, which persist into
+            // the borderless-fullscreen popup window and black out fullscreen video.
+            var darkHr = dwm.DwmSetWindowAttribute(hwnd, DWMWA_USE_IMMERSIVE_DARK_MODE, IntByReference(1), 4)
+            if (darkHr != S_OK) {
+                darkHr = dwm.DwmSetWindowAttribute(hwnd, DWMWA_USE_IMMERSIVE_DARK_MODE_PRE_20H1, IntByReference(1), 4)
+            }
+            DesktopRuntimeLog.info("window chrome polish hr: darkMode=$darkHr (0=S_OK)")
+        }.onFailure {
+            DesktopRuntimeLog.warn("window chrome polish failed ${it::class.simpleName}:${it.message}")
+        }
+    }
+}