Issue #21236: Fixes empty tray visibility logic

This is a bug we noticed after landing search term grouping.

An adapter can submit an empty list of items to the `ConcatAdapter`
early. This has the side-effect of triggering our `observeFirstInsert`
too soon and therefore updating the visibility to show the empty tray
placeholder and never switches back.

Our solution is to keep a constant observer on the adapter so we can
perform the visibility check on every insert/remove.

Co-authored-by: Roger Yang <royang@mozilla.com>

app/src/main/java/org/mozilla/fenix/tabstray/TrayPagerAdapter.kt

@@ -106,6 +106,14 @@ class TrayPagerAdapter(
         }
     }
 
+    override fun onViewAttachedToWindow(holder: AbstractPageViewHolder) {
+        holder.attachedToWindow()
+    }
+
+    override fun onViewDetachedFromWindow(holder: AbstractPageViewHolder) {
+        holder.detachedFromWindow()
+    }
+
     override fun getItemCount(): Int = TRAY_TABS_COUNT
 
     companion object {

app/src/main/java/org/mozilla/fenix/tabstray/viewholders/AbstractBrowserPageViewHolder.kt

@@ -13,8 +13,8 @@ import androidx.recyclerview.widget.RecyclerView
 import org.mozilla.fenix.R
 import org.mozilla.fenix.tabstray.TabsTrayInteractor
 import org.mozilla.fenix.tabstray.TabsTrayStore
+import org.mozilla.fenix.tabstray.TrayPagerAdapter
 import org.mozilla.fenix.tabstray.browser.AbstractBrowserTrayList
-import org.mozilla.fenix.tabstray.ext.observeFirstInsert
 
 /**
  * A shared view holder for browser tabs tray list.
@@ -27,6 +27,9 @@ abstract class AbstractBrowserPageViewHolder(
 
     private val trayList: AbstractBrowserTrayList = itemView.findViewById(R.id.tray_list_item)
     private val emptyList: TextView = itemView.findViewById(R.id.tab_tray_empty_view)
+    private var adapterObserver: RecyclerView.AdapterDataObserver? = null
+    private var adapterRef: RecyclerView.Adapter<out RecyclerView.ViewHolder>? = null
+
     abstract val emptyStringText: String
 
     init {
@@ -48,14 +51,53 @@ abstract class AbstractBrowserPageViewHolder(
         adapter: RecyclerView.Adapter<out RecyclerView.ViewHolder>,
         layoutManager: RecyclerView.LayoutManager
     ) {
-        adapter.observeFirstInsert {
-            updateTrayVisibility(adapter.itemCount)
-        }
+        adapterRef = adapter
+
         scrollToTab(adapter, layoutManager)
+
         trayList.layoutManager = layoutManager
         trayList.adapter = adapter
     }
 
+    /**
+     * When the [RecyclerView.Adapter] is attached to the window we register a data observer to
+     * always check whether to call [updateTrayVisibility].
+     *
+     * We keep a constant observer instead of using [RecyclerView.Adapter.observeFirstInsert],
+     * because some adapters can insert empty lists and trigger the one-shot observer too soon.
+     *
+     * See also [AbstractPageViewHolder.attachedToWindow].
+     */
+    override fun attachedToWindow() {
+        adapterRef?.let { adapter ->
+            adapterObserver = object : RecyclerView.AdapterDataObserver() {
+                override fun onItemRangeInserted(positionStart: Int, itemCount: Int) {
+                    updateTrayVisibility(adapter.itemCount)
+                }
+
+                override fun onItemRangeRemoved(positionstart: Int, itemcount: Int) {
+                    updateTrayVisibility(adapter.itemCount)
+                }
+            }
+            adapterObserver?.let {
+                adapter.registerAdapterDataObserver(it)
+            }
+        }
+    }
+
+    /**
+     * [RecyclerView.AdapterDataObserver]s are responsible to be unregistered when they are done,
+     * so we do that here when our [TrayPagerAdapter] page is detached from the window.
+     *
+     * See also [AbstractPageViewHolder.detachedFromWindow].
+     */
+    override fun detachedFromWindow() {
+        adapterObserver?.let {
+            adapterRef?.unregisterAdapterDataObserver(it)
+            adapterObserver = null
+        }
+    }
+
     private fun updateTrayVisibility(size: Int) {
         if (size == 0) {
             trayList.visibility = GONE

app/src/main/java/org/mozilla/fenix/tabstray/viewholders/AbstractPageViewHolder.kt

@@ -15,7 +15,21 @@ abstract class AbstractPageViewHolder constructor(
     val containerView: View
 ) : RecyclerView.ViewHolder(containerView) {
 
+    /**
+     * Invoked when the nested [RecyclerView.Adapter] is bound to the [RecyclerView.ViewHolder].
+     */
     abstract fun bind(
         adapter: RecyclerView.Adapter<out RecyclerView.ViewHolder>
     )
+
+    /**
+     * Invoked when the [RecyclerView.ViewHolder] is attached from the window. This could have
+     * previously been bound and is now attached again.
+     */
+    abstract fun attachedToWindow()
+
+    /**
+     * Invoked when the [RecyclerView.ViewHolder] is detached from the window.
+     */
+    abstract fun detachedFromWindow()
 }

app/src/main/java/org/mozilla/fenix/tabstray/viewholders/SyncedTabsPageViewHolder.kt

@@ -27,6 +27,9 @@ class SyncedTabsPageViewHolder(
         binding.syncedTabsTrayLayout.tabsTrayStore = tabsTrayStore
     }
 
+    override fun detachedFromWindow() = Unit // no-op
+    override fun attachedToWindow() = Unit // no-op
+
     companion object {
         const val LAYOUT_ID = R.layout.component_sync_tabs_tray_layout
     }

app/src/test/java/org/mozilla/fenix/tabstray/viewholders/AbstractBrowserPageViewHolderTest.kt

@@ -41,6 +41,7 @@ class AbstractBrowserPageViewHolderTest {
         val emptyList: TextView = itemView.findViewById(R.id.tab_tray_empty_view)
 
         viewHolder.bind(adapter)
+        viewHolder.attachedToWindow()
 
         adapter.updateTabs(
             Tabs(
@@ -65,6 +66,7 @@ class AbstractBrowserPageViewHolderTest {
         val emptyList: TextView = itemView.findViewById(R.id.tab_tray_empty_view)
 
         viewHolder.bind(adapter)
+        viewHolder.attachedToWindow()
 
         adapter.updateTabs(
             Tabs(