Issue #18934: Do not nullify adapter on window detached

Previously, to fix a memory leak, we were removing the adapter reference
entirely in order to have the `onDetachedFromRecyclerView` callback
invoked. This causes a side-effect where we can no longer reference the
adapter any more when we re-attach.

The simpler solution is to just invoke the needed callback directly
instead.

app/src/main/java/org/mozilla/fenix/tabstray/browser/BaseBrowserTrayList.kt

@@ -6,6 +6,7 @@ package org.mozilla.fenix.tabstray.browser
 
 import android.content.Context
 import android.util.AttributeSet
+import androidx.annotation.VisibleForTesting
 import androidx.recyclerview.widget.RecyclerView
 import mozilla.components.feature.tabs.tabstray.TabsFeature
 import org.mozilla.fenix.ext.components
@@ -82,14 +83,15 @@ abstract class BaseBrowserTrayList @JvmOverloads constructor(
         touchHelper.attachToRecyclerView(this)
     }
 
-    override fun onDetachedFromWindow() {
+    @VisibleForTesting
+    public override fun onDetachedFromWindow() {
         super.onDetachedFromWindow()
 
         tabsFeature.stop()
         swipeToDelete.stop()
 
-        // Release the adapter so that `onDetachedFromRecyclerView` will be called in the adapter.
-        adapter = null
+        // Notify the adapter that it is released from the view preemptively.
+        adapter?.onDetachedFromRecyclerView(this)
 
         touchHelper.attachToRecyclerView(null)
     }

app/src/test/java/org/mozilla/fenix/tabstray/browser/BaseBrowserTrayListTest.kt

@@ -0,0 +1,35 @@
+/* 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.tabstray.browser
+
+import android.content.Context
+import io.mockk.mockk
+import io.mockk.verify
+import mozilla.components.support.test.robolectric.testContext
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.mozilla.fenix.helpers.FenixRobolectricTestRunner
+import org.mozilla.fenix.tabstray.TabsTrayStore
+
+@RunWith(FenixRobolectricTestRunner::class)
+class BaseBrowserTrayListTest {
+
+    @Test
+    fun `WHEN recyclerview detaches from window THEN notify adapter`() {
+        val trayList = TestBaseBrowserTrayList(testContext)
+        val adapter = mockk<BrowserTabsAdapter>(relaxed = true)
+
+        trayList.adapter = adapter
+        trayList.tabsTrayStore = TabsTrayStore()
+
+        trayList.onDetachedFromWindow()
+
+        verify { adapter.onDetachedFromRecyclerView(trayList) }
+    }
+
+    class TestBaseBrowserTrayList(context: Context) : BaseBrowserTrayList(context) {
+        override val configuration = Configuration(BrowserTabType.PRIVATE)
+    }
+}