[fenix] For https://github.com/mozilla-mobile/fenix/issues/27016 - Ensure smooth search UX after the MR onboarding is closed

If the app is opened from the search widget and the MR onboarding is shown then
the backstack will have the following structure:
- root, homeFragment, searchDialogFragment, onboardingFragment
as opposed to otherwise
- root, homeFragment, searchDialogFragment.

This patch allows to avoid the MR onboarding fragment causing the
SearchDialogFragment to not know that below it is the HomeFragment and
consequently not applying transparency or propagate user touches to the parent
Activity.

app/src/main/java/org/mozilla/fenix/search/SearchDialogFragment.kt

@@ -5,6 +5,7 @@
 package org.mozilla.fenix.search
 
 import android.Manifest
+import android.annotation.SuppressLint
 import android.app.Activity
 import android.app.Dialog
 import android.content.Context
@@ -24,6 +25,7 @@ import android.view.ViewStub
 import android.view.WindowManager
 import android.view.accessibility.AccessibilityEvent
 import android.view.inputmethod.InputMethodManager
+import androidx.annotation.VisibleForTesting
 import androidx.appcompat.app.AlertDialog
 import androidx.appcompat.app.AppCompatDialogFragment
 import androidx.appcompat.content.res.AppCompatResources
@@ -35,9 +37,10 @@ import androidx.core.graphics.drawable.toDrawable
 import androidx.core.net.toUri
 import androidx.core.view.isVisible
 import androidx.lifecycle.lifecycleScope
+import androidx.navigation.NavBackStackEntry
+import androidx.navigation.NavGraph
 import androidx.navigation.fragment.findNavController
 import androidx.navigation.fragment.navArgs
-import kotlinx.coroutines.flow.collect
 import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.launch
 import mozilla.components.browser.domains.autocomplete.ShippedDomainsProvider
@@ -202,7 +205,7 @@ class SearchDialogFragment : AppCompatDialogFragment(), UserInteractionHandler {
         )
 
         val fromHomeFragment =
-            findNavController().previousBackStackEntry?.destination?.id == R.id.homeFragment
+            getPreviousDestination()?.destination?.id == R.id.homeFragment
 
         toolbarView = ToolbarView(
             requireContext(),
@@ -252,7 +255,7 @@ class SearchDialogFragment : AppCompatDialogFragment(), UserInteractionHandler {
 
         requireComponents.core.engine.speculativeCreateSession(isPrivate)
 
-        when (findNavController().previousBackStackEntry?.destination?.id) {
+        when (getPreviousDestination()?.destination?.id) {
             R.id.homeFragment -> {
                 // When displayed above home, dispatches the touch events to scrim area to the HomeFragment
                 binding.searchWrapper.background = ColorDrawable(Color.TRANSPARENT)
@@ -301,7 +304,7 @@ class SearchDialogFragment : AppCompatDialogFragment(), UserInteractionHandler {
         setupConstraints(view)
 
         // When displayed above browser or home screen, dismisses keyboard when touching scrim area
-        when (findNavController().previousBackStackEntry?.destination?.id) {
+        when (getPreviousDestination()?.destination?.id) {
             R.id.browserFragment, R.id.homeFragment -> {
                 binding.searchWrapper.setOnTouchListener { _, _ ->
                     binding.searchWrapper.hideKeyboard()
@@ -904,6 +907,37 @@ class SearchDialogFragment : AppCompatDialogFragment(), UserInteractionHandler {
         }
     }
 
+    /**
+     * Gets the previous visible [NavBackStackEntry].
+     * This skips over any [NavBackStackEntry] that is associated with a [NavGraph] or refers to this
+     * class as a navigation destination.
+     */
+    @VisibleForTesting
+    @SuppressLint("RestrictedApi")
+    internal fun getPreviousDestination(): NavBackStackEntry? {
+        // This duplicates the platform functionality for "previousBackStackEntry" but additionally skips this entry.
+
+        val descendingEntries = findNavController().backStack.descendingIterator()
+        // Throw the topmost destination away.
+        if (descendingEntries.hasNext()) {
+            descendingEntries.next()
+        }
+
+        while (descendingEntries.hasNext()) {
+            val entry = descendingEntries.next()
+            // Using the canonicalName is safer - see https://github.com/mozilla-mobile/android-components/pull/10810
+            // simpleName is used as a backup to avoid the not null assertion (!!) operator.
+            val currentClassName = this::class.java.canonicalName?.substringAfterLast('.')
+                ?: this::class.java.simpleName
+
+            // Throw this entry away if it's the current top and ignore returning the base nav graph.
+            if (entry.destination !is NavGraph && !entry.destination.displayName.contains(currentClassName, true)) {
+                return entry
+            }
+        }
+        return null
+    }
+
     companion object {
         private const val TAP_INCREASE_DPS = 8
         private const val QR_FRAGMENT_TAG = "MOZAC_QR_FRAGMENT"

app/src/test/java/org/mozilla/fenix/search/SearchDialogFragmentTest.kt

@@ -0,0 +1,92 @@
+/* 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.search
+
+import androidx.fragment.app.Fragment
+import androidx.navigation.NavBackStackEntry
+import androidx.navigation.NavController
+import androidx.navigation.fragment.findNavController
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.mockkStatic
+import io.mockk.unmockkStatic
+import org.junit.After
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertSame
+import org.junit.Before
+import org.junit.Test
+import java.util.ArrayDeque
+
+internal class SearchDialogFragmentTest {
+    private val navController: NavController = mockk()
+    private val fragment = SearchDialogFragment()
+
+    @Before
+    fun setup() {
+        mockkStatic("androidx.navigation.fragment.FragmentKt")
+        every { any<Fragment>().findNavController() } returns navController
+    }
+
+    @After
+    fun teardown() {
+        unmockkStatic("androidx.navigation.fragment.FragmentKt")
+    }
+
+    @Test
+    fun `GIVEN this is the only visible fragment WHEN asking for the previous destination THEN return null`() {
+        every { navController.backStack } returns ArrayDeque(listOf(getDestination(fragmentName)))
+
+        assertNull(fragment.getPreviousDestination())
+    }
+
+    @Test
+    fun `GIVEN this and FragmentB on top of this are visible WHEN asking for the previous destination THEN return null`() {
+        every { navController.backStack } returns ArrayDeque(
+            listOf(
+                getDestination(fragmentName),
+                getDestination("FragmentB"),
+            ),
+        )
+
+        assertNull(fragment.getPreviousDestination())
+    }
+
+    @Test
+    fun `GIVEN FragmentA, this and FragmentB are visible WHEN asking for the previous destination THEN return FragmentA`() {
+        val fragmentADestination = getDestination("FragmentA")
+        every { navController.backStack } returns ArrayDeque(
+            listOf(
+                fragmentADestination,
+                getDestination(fragmentName),
+                getDestination("FragmentB"),
+            ),
+        )
+
+        assertSame(fragmentADestination, fragment.getPreviousDestination())
+    }
+
+    @Test
+    fun `GIVEN FragmentA and this on top of it are visible WHEN asking for the previous destination THEN return FragmentA`() {
+        val fragmentADestination = getDestination("FragmentA")
+        every { navController.backStack } returns ArrayDeque(
+            listOf(
+                fragmentADestination,
+                getDestination(fragmentName),
+            ),
+        )
+
+        assertSame(fragmentADestination, fragment.getPreviousDestination())
+    }
+}
+
+private val fragmentName = SearchDialogFragment::class.java.canonicalName?.substringAfterLast('.')!!
+
+private fun getDestination(destinationName: String): NavBackStackEntry {
+    return mockk {
+        every { destination } returns mockk {
+            every { displayName } returns "test.id/$destinationName"
+        }
+    }
+}