Fix stale thread selection in chat list

See [here](https://github.com/signalapp/Signal-iOS/pull/5432).

Signal/src/ViewControllers/HomeView/Chat List/ChatListViewController+Actions.swift

@@ -22,7 +22,6 @@ extension ChatListViewController {
         if let previousIndexPath = renderState.indexPath(beforeThread: currentThread),
            let thread = self.thread(forIndexPath: previousIndexPath) {
             self.present(thread, action: .compose, animated: true)
-            tableView.selectRow(at: previousIndexPath, animated: true, scrollPosition: .none)
         }
     }
 
@@ -41,7 +40,6 @@ extension ChatListViewController {
         if let nextIndexPath = renderState.indexPath(afterThread: currentThread),
            let thread = self.thread(forIndexPath: nextIndexPath) {
             self.present(thread, action: .compose, animated: true)
-            tableView.selectRow(at: nextIndexPath, animated: true, scrollPosition: .none)
         }
     }
 }

Signal/src/ViewControllers/HomeView/Chat List/ChatListViewController+Helpers.swift

@@ -146,8 +146,6 @@ public extension ChatListViewController {
             owsFailDebug("Missing threadViewModel.")
             return nil
         }
-        self.lastViewedThread = threadViewModel.threadRecord
-
         let vc = ConversationViewController(threadViewModel: threadViewModel,
                                             action: .none,
                                             focusMessageId: nil)

Signal/src/ViewControllers/HomeView/Chat List/ChatListViewController.h

@@ -29,7 +29,9 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)unarchiveSelectedConversation;
 
 @property (nonatomic, readonly) CLVViewState *viewState;
-@property (nonatomic) TSThread *lastViewedThread;
+
+/// Used to update the selected cell for split view and maintain scroll positions for reappearing collapsed views.
+- (void)updateLastViewedThread:(TSThread *)thread animated:(BOOL)animated;
 
 // For use by Swift extension.
 - (void)updateBarButtonItems;

Signal/src/ViewControllers/HomeView/Chat List/ChatListViewController.m

@@ -53,6 +53,8 @@ NSString *const kArchiveButtonPseudoGroup = @"kArchiveButtonPseudoGroup";
 
 @property (nonatomic) BOOL hasEverPresentedExperienceUpgrade;
 
+@property (nonatomic, nullable) TSThread *lastViewedThread;
+
 @end
 
 #pragma mark -
@@ -696,6 +698,12 @@ NSString *const kArchiveButtonPseudoGroup = @"kArchiveButtonPseudoGroup";
     [self.searchResultsController viewWillDisappear:animated];
 }
 
+- (void)updateLastViewedThread:(TSThread *)thread animated:(BOOL)animated
+{
+    self.lastViewedThread = thread;
+    [self ensureSelectedThread:thread animated:animated];
+}
+
 #pragma mark -
 
 - (void)pullToRefreshPerformed:(UIRefreshControl *)refreshControl

Signal/src/ViewControllers/HomeView/Chat List/ChatListViewController.swift

@@ -497,6 +497,32 @@ public extension ChatListViewController {
         navigationController.setViewControllers(viewControllers, animated: false)
         presentFormSheet(navigationController, animated: true, completion: completion)
     }
+
+    /// Verifies that the currently selected cell matches the provided thread.
+    /// If it does or if the user's in multi-select: Do nothing.
+    /// If it doesn't: Select the first cell matching the provided thread, if one exists. Otherwise, deselect the current row.
+    @objc
+    func ensureSelectedThread(_ targetThread: TSThread, animated: Bool) {
+        // Ignore any updates if we're in multiselect mode. I don't think this can happen,
+        // but if it does let's avoid stepping over the user's manual selection.
+        let currentSelection = tableView.indexPathsForSelectedRows ?? []
+        guard viewState.multiSelectState.isActive == false, currentSelection.count < 2 else {
+            return
+        }
+
+        let currentlySelectedThread = currentSelection.first.flatMap {
+            self.tableDataSource.thread(forIndexPath: $0, expectsSuccess: false)
+        }
+
+        if currentlySelectedThread?.uniqueId != targetThread.uniqueId {
+            if let targetPath = tableDataSource.renderState.indexPath(forUniqueId: targetThread.uniqueId) {
+                tableView.selectRow(at: targetPath, animated: animated, scrollPosition: .none)
+                tableView.scrollToRow(at: targetPath, at: .none, animated: animated)
+            } else if let stalePath = currentSelection.first {
+                tableView.deselectRow(at: stalePath, animated: animated)
+            }
+        }
+    }
 }
 
 extension ChatListViewController: BadgeExpirationSheetDelegate {

Signal/src/ViewControllers/HomeView/ConversationSplitViewController.swift

@@ -184,7 +184,7 @@ class ConversationSplitViewController: UISplitViewController, ConversationSplit
 
         // Update the last viewed thread on the conversation list so it
         // can maintain its scroll position when navigating back.
-        homeVC.chatListViewController.lastViewedThread = thread
+        homeVC.chatListViewController.updateLastViewedThread(thread, animated: animated)
 
         let threadViewModel = databaseStorage.read {
             return ThreadViewModel(thread: thread,