Fix CFv2 initial scrolling bugs.

app/src/main/java/androidx/recyclerview/widget/ConversationLayoutManager.kt

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2023 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package androidx.recyclerview.widget
+
+import android.content.Context
+import org.signal.core.util.logging.Log
+
+/**
+ * Variation of a vertical, reversed [LinearLayoutManager] that makes specific assumptions in how it will
+ * be used by Conversation view to support easier scrolling to the initial start position.
+ *
+ * Primarily, it assumes that an initial scroll to position call will always happen and that the implementation
+ * of [LinearLayoutManager] remains unchanged with respect to how it assigns [mPendingScrollPosition] and
+ * [mPendingScrollPositionOffset] in [LinearLayoutManager.scrollToPositionWithOffset] and how it always clears
+ * the pending state variables in every call to [LinearLayoutManager.onLayoutCompleted].
+ *
+ * The assumptions are necessary to force the requested scroll position/layout to occur even if the request
+ * happens prior to the data source populating the recycler view/adapter.
+ */
+class ConversationLayoutManager(context: Context) : LinearLayoutManager(context, RecyclerView.VERTICAL, true) {
+
+  private var afterScroll: (() -> Unit)? = null
+
+  override fun supportsPredictiveItemAnimations(): Boolean {
+    return false
+  }
+
+  /**
+   * Scroll to the desired position and be notified when the layout manager has completed the request
+   * via [afterScroll] callback.
+   */
+  fun scrollToPositionWithOffset(position: Int, offset: Int, afterScroll: () -> Unit) {
+    this.afterScroll = afterScroll
+    super.scrollToPositionWithOffset(position, offset)
+  }
+
+  /**
+   * If a scroll to position request is made and a layout pass occurs prior to the list being populated with via the data source,
+   * the base implementation clears the request as if it was never made.
+   *
+   * This override will capture the pending scroll position and offset, determine if the scroll request was satisfied, and
+   * re-request the scroll to position to force another attempt if not satisfied.
+   *
+   * A pending scroll request will be re-requested if the pending scroll position is outside the bounds of the current known size of
+   * items in the list.
+   */
+  override fun onLayoutCompleted(state: RecyclerView.State?) {
+    val pendingScrollPosition = mPendingScrollPosition
+    val pendingScrollOffset = mPendingScrollPositionOffset
+
+    val reRequestPendingPosition = pendingScrollPosition >= (state?.mItemCount ?: 0)
+
+    // Base implementation always clears mPendingScrollPosition+mPendingScrollPositionOffset
+    super.onLayoutCompleted(state)
+
+    // Re-request scroll to position request if necessary thus forcing mPendingScrollPosition+mPendingScrollPositionOffset to be re-assigned
+    if (reRequestPendingPosition) {
+      Log.d(TAG, "Re-requesting pending scroll position: $pendingScrollPosition offset: $pendingScrollOffset")
+      if (pendingScrollOffset != INVALID_OFFSET) {
+        scrollToPositionWithOffset(pendingScrollPosition, pendingScrollOffset)
+      } else {
+        scrollToPosition(pendingScrollPosition)
+      }
+    } else {
+      afterScroll?.invoke()
+      afterScroll = null
+    }
+  }
+
+  companion object {
+    private val TAG = Log.tag(ConversationLayoutManager::class.java)
+  }
+}