[fenix] Closes https://github.com/mozilla-mobile/fenix/issues/21759: Do not render home fragment when launching to tab

Co-authored-by: Sebastian Kaspari <s.kaspari@gmail.com>

app/src/main/java/org/mozilla/fenix/HomeActivity.kt

@@ -132,7 +132,7 @@ import java.lang.ref.WeakReference
  * - browser screen
  */
 @OptIn(ExperimentalCoroutinesApi::class)
-@SuppressWarnings("TooManyFunctions", "LargeClass", "LongParameterList")
+@SuppressWarnings("TooManyFunctions", "LargeClass", "LongParameterList", "LongMethod")
 open class HomeActivity : LocaleAwareAppCompatActivity(), NavHostActivity {
     // DO NOT MOVE ANYTHING ABOVE THIS, GETTING INIT TIME IS CRITICAL
     // we need to store startup timestamp for warm startup. we cant directly store
@@ -229,12 +229,18 @@ open class HomeActivity : LocaleAwareAppCompatActivity(), NavHostActivity {
             it.start()
         }
 
-        if (!shouldStartOnHome() &&
-            shouldNavigateBrowserFragmentOnColdStart(savedInstanceState)
-        ) {
+        // Unless the activity is recreated, navigate to home first (without rendering it)
+        // to add it to the back stack.
+        if (savedInstanceState == null) {
+            navigateToHome()
+        }
+
+        if (!shouldStartOnHome() && shouldNavigateToBrowserOnColdStart(savedInstanceState)) {
             navigateToBrowserOnColdStart()
-        } else if (FeatureFlags.showStartOnHomeSettings) {
-            components.analytics.metrics.track(Event.StartOnHomeEnterHomeScreen)
+        } else {
+            if (FeatureFlags.showStartOnHomeSettings) {
+                components.analytics.metrics.track(Event.StartOnHomeEnterHomeScreen)
+            }
         }
 
         Performance.processIntentIfPerformanceTest(intent, this)
@@ -879,13 +885,16 @@ open class HomeActivity : LocaleAwareAppCompatActivity(), NavHostActivity {
     open fun navigateToBrowserOnColdStart() {
         // Normal tabs + cold start -> Should go back to browser if we had any tabs open when we left last
         // except for PBM + Cold Start there won't be any tabs since they're evicted so we never will navigate
-        if (settings().shouldReturnToBrowser &&
-            !browsingModeManager.mode.isPrivate
-        ) {
+        if (settings().shouldReturnToBrowser && !browsingModeManager.mode.isPrivate) {
+            // Navigate to home first (without rendering it) to add it to the back stack.
             openToBrowser(BrowserDirection.FromGlobal, null)
         }
     }
 
+    open fun navigateToHome() {
+        navHost.navController.navigate(NavGraphDirections.actionStartupHome())
+    }
+
     override fun attachBaseContext(base: Context) {
         base.components.strictMode.resetAfter(StrictMode.allowThreadDiskReads()) {
             super.attachBaseContext(base)
@@ -1001,7 +1010,7 @@ open class HomeActivity : LocaleAwareAppCompatActivity(), NavHostActivity {
     @VisibleForTesting
     internal fun getSettings(): Settings = settings()
 
-    private fun shouldNavigateBrowserFragmentOnColdStart(savedInstanceState: Bundle?): Boolean {
+    private fun shouldNavigateToBrowserOnColdStart(savedInstanceState: Bundle?): Boolean {
         return isActivityColdStarted(intent, savedInstanceState) &&
             !externalSourceIntentProcessors.any {
                 it.process(

app/src/main/java/org/mozilla/fenix/StartupFragment.kt

@@ -0,0 +1,19 @@
+/* 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 http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix
+
+import androidx.fragment.app.Fragment
+import org.mozilla.fenix.home.HomeFragment
+
+/**
+ * This empty fragment serves as a start destination in our navigation
+ * graph. It contains no layout and is fast to create compared to our
+ * [HomeFragment], which would otherwise be the start destination.
+ *
+ * When our [HomeActivity] is created we make a decision which fragment
+ * to navigate to, which makes sure we only render the [HomeFragment]
+ * as needed.
+ */
+class StartupFragment : Fragment()

app/src/main/java/org/mozilla/fenix/customtabs/ExternalAppBrowserActivity.kt

@@ -53,6 +53,10 @@ open class ExternalAppBrowserActivity : HomeActivity() {
         // No-op for external app
     }
 
+    override fun navigateToHome() {
+        // No-op for external app
+    }
+
     override fun handleNewIntent(intent: Intent) {
         // No-op for external app
     }

app/src/main/res/navigation/nav_graph.xml

@@ -3,7 +3,13 @@
     xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:id="@+id/nav_graph"
-    app:startDestination="@id/homeFragment">
+    app:startDestination="@id/startupFragment">
+
+    <action
+        android:id="@+id/action_startup_home"
+        app:destination="@id/homeFragment"
+        app:popUpTo="@id/startupFragment"
+        app:popUpToInclusive="true" />
 
     <action
         android:id="@+id/action_global_home"
@@ -157,6 +163,11 @@
             app:argType="string" />
     </dialog>
 
+    <fragment
+        android:id="@+id/startupFragment"
+        android:name="org.mozilla.fenix.StartupFragment">
+    </fragment>
+
     <fragment
         android:id="@+id/homeFragment"
         android:name="org.mozilla.fenix.home.HomeFragment"