fix color scheme for custom tabs

apps/weblibre/lib/features/geckoview/features/browser/domain/services/engine_settings_replication.dart

@@ -216,12 +216,11 @@ class EngineSettingsReplicationService
                 settings.globalPrivacyControlEnabled,
               );
             }
-            if (previous.value?.preferredColorScheme !=
-                settings.preferredColorScheme) {
-              await _service.preferredColorScheme(
-                settings.preferredColorScheme,
-              );
-            }
+            // Note: preferredColorScheme is intentionally NOT replicated from the
+            // engine settings here. The app theme (generalSettings.themeMode) is the
+            // sole source of truth for the color scheme; see the themeMode listener
+            // above. EngineSettings.preferredColorScheme is vestigial and always
+            // `system`, so replicating it would clobber the real theme (issue #436).
             if (previous.value?.cookieBannerHandlingMode !=
                 settings.cookieBannerHandlingMode) {
               await _service.cookieBannerHandlingMode(

packages/flutter_mozilla_components/android/src/main/kotlin/eu/weblibre/flutter_mozilla_components/ColorSchemePreference.kt

@@ -0,0 +1,78 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package eu.weblibre.flutter_mozilla_components
+
+import android.content.Context
+import android.content.SharedPreferences
+import androidx.appcompat.app.AppCompatDelegate
+import androidx.core.content.edit
+import androidx.preference.PreferenceManager
+import mozilla.components.concept.engine.mediaquery.PreferredColorScheme
+
+/**
+ * Persists the user's preferred color scheme in the default [SharedPreferences].
+ *
+ * Flutter remains the source of truth for the theme, but Custom Tab / PWA sessions
+ * run in a native, non-Flutter activity ([eu.weblibre.flutter_mozilla_components.activities.ExternalAppBrowserActivity]).
+ * When such a session cold-starts the app, the engine is built before Flutter ever
+ * runs, so it cannot ask Flutter for the scheme. Without a persisted value the engine
+ * falls back to a hardcoded default and reports the wrong `prefers-color-scheme` to
+ * web content (see issue #436). This lets the native side resolve the last known
+ * choice on cold start.
+ */
+object ColorSchemePreference {
+    private const val PREF_KEY = "weblibre_preferred_color_scheme"
+
+    private const val VALUE_SYSTEM = "system"
+    private const val VALUE_LIGHT = "light"
+    private const val VALUE_DARK = "dark"
+
+    /**
+     * Fallback used before Flutter has written a value (e.g. the very first
+     * Custom Tab / PWA launch). Must match WebLibre's default app theme
+     * (`ThemeMode.dark`, general_settings.dart) so a direct launch doesn't briefly
+     * disagree with the app's default.
+     */
+    private val DEFAULT = PreferredColorScheme.Dark
+
+    fun read(prefs: SharedPreferences): PreferredColorScheme =
+        when (prefs.getString(PREF_KEY, null)) {
+            VALUE_SYSTEM -> PreferredColorScheme.System
+            VALUE_LIGHT -> PreferredColorScheme.Light
+            VALUE_DARK -> PreferredColorScheme.Dark
+            else -> DEFAULT
+        }
+
+    fun read(context: Context): PreferredColorScheme =
+        read(PreferenceManager.getDefaultSharedPreferences(context))
+
+    fun write(prefs: SharedPreferences, scheme: PreferredColorScheme) {
+        prefs.edit {
+            putString(
+                PREF_KEY,
+                when (scheme) {
+                    PreferredColorScheme.Light -> VALUE_LIGHT
+                    PreferredColorScheme.Dark -> VALUE_DARK
+                    PreferredColorScheme.System -> VALUE_SYSTEM
+                },
+            )
+        }
+    }
+
+    /**
+     * Maps the persisted scheme to an [AppCompatDelegate] night mode so native
+     * activities (Custom Tab / PWA) render their window chrome in the mode the user
+     * selected in WebLibre, instead of always following the system. This avoids the
+     * window flashing dark before the page paints when WebLibre is forced to light.
+     */
+    fun nightMode(context: Context): Int =
+        when (read(context)) {
+            PreferredColorScheme.Light -> AppCompatDelegate.MODE_NIGHT_NO
+            PreferredColorScheme.Dark -> AppCompatDelegate.MODE_NIGHT_YES
+            PreferredColorScheme.System -> AppCompatDelegate.MODE_NIGHT_FOLLOW_SYSTEM
+        }
+}

packages/flutter_mozilla_components/android/src/main/kotlin/eu/weblibre/flutter_mozilla_components/activities/ExternalAppBrowserActivity.kt

@@ -13,6 +13,7 @@ import android.util.Log
 import androidx.activity.addCallback
 import androidx.appcompat.app.AppCompatActivity
 import androidx.core.view.WindowCompat
+import eu.weblibre.flutter_mozilla_components.ColorSchemePreference
 import eu.weblibre.flutter_mozilla_components.ExternalAppBrowserFragment
 import eu.weblibre.flutter_mozilla_components.GlobalComponents
 import eu.weblibre.flutter_mozilla_components.PwaConstants
@@ -76,6 +77,12 @@ open class ExternalAppBrowserActivity : AppCompatActivity() {
         get() = intent?.getStringExtra(EXTRA_WEB_APP_MANIFEST_URL)
 
     override fun onCreate(savedInstanceState: Bundle?) {
+        // Match the window chrome (status/nav bar + pre-paint background) to the
+        // user's WebLibre color scheme rather than just the system mode, so a
+        // cold-started Custom Tab / PWA doesn't flash dark when WebLibre is light.
+        // Set before super.onCreate so the correct mode is applied without a recreate.
+        delegate.localNightMode = ColorSchemePreference.nightMode(this)
+
         super.onCreate(savedInstanceState)
 
         onBackPressedDispatcher.addCallback(this) {

packages/flutter_mozilla_components/android/src/main/kotlin/eu/weblibre/flutter_mozilla_components/api/GeckoEngineSettingsApiImpl.kt

@@ -8,6 +8,7 @@ package eu.weblibre.flutter_mozilla_components.api
 
 import androidx.core.content.edit
 import androidx.preference.PreferenceManager
+import eu.weblibre.flutter_mozilla_components.ColorSchemePreference
 import eu.weblibre.flutter_mozilla_components.GlobalComponents
 import eu.weblibre.flutter_mozilla_components.R
 import eu.weblibre.flutter_mozilla_components.pigeons.AppLinksMode
@@ -336,6 +337,13 @@ class GeckoEngineSettingsApiImpl : GeckoEngineSettingsApi {
         }
         if(settings.preferredColorScheme != null) {
             components.core.engine.settings.preferredColorScheme = components.core.engineSettings.preferredColorScheme
+            // Persist so cold-started Custom Tab / PWA sessions resolve the right
+            // scheme before Flutter runs. Only done here (the runtime path driven by
+            // the app theme), not in setDefaultSettings, so the vestigial engine
+            // settings payload can't clobber the real theme. See issue #436.
+            components.core.engineSettings.preferredColorScheme?.let { scheme ->
+                ColorSchemePreference.write(components.core.prefs, scheme)
+            }
             reloadSession = true
         }
         if(settings.cookieBannerHandlingMode != null) {

packages/flutter_mozilla_components/android/src/main/kotlin/eu/weblibre/flutter_mozilla_components/components/Core.kt

@@ -11,6 +11,7 @@ import android.content.SharedPreferences
 import android.os.Environment
 import androidx.core.content.ContextCompat
 import androidx.preference.PreferenceManager
+import eu.weblibre.flutter_mozilla_components.ColorSchemePreference
 import eu.weblibre.flutter_mozilla_components.Components
 import eu.weblibre.flutter_mozilla_components.interceptor.AppRequestInterceptor
 import eu.weblibre.flutter_mozilla_components.services.DownloadService
@@ -47,7 +48,6 @@ import mozilla.components.concept.engine.Engine
 import mozilla.components.concept.engine.EngineSession
 import mozilla.components.concept.engine.EngineSession.TrackingProtectionPolicy
 import mozilla.components.concept.engine.fission.WebContentIsolationStrategy
-import mozilla.components.concept.engine.mediaquery.PreferredColorScheme
 import mozilla.components.concept.fetch.Client
 import mozilla.components.feature.addons.AddonManager
 import mozilla.components.feature.addons.amo.AMOAddonsProvider
@@ -122,7 +122,10 @@ class Core(
             //fingerprintingProtectionPrivateBrowsing
             httpsOnlyMode = Engine.HttpsOnlyMode.ENABLED,
             globalPrivacyControlEnabled = true,
-            preferredColorScheme = PreferredColorScheme.Dark,
+            // Resolve the last persisted choice so cold-started Custom Tab / PWA
+            // sessions report the correct `prefers-color-scheme` before Flutter
+            // (the source of truth) runs. Defaults to System. See issue #436.
+            preferredColorScheme = ColorSchemePreference.read(prefs),
             cookieBannerHandlingMode = EngineSession.CookieBannerHandlingMode.REJECT_ALL,
             cookieBannerHandlingModePrivateBrowsing = EngineSession.CookieBannerHandlingMode.REJECT_ALL,
             cookieBannerHandlingGlobalRules = true,