Only mark messages read in conversation view if visible.

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -3798,6 +3798,15 @@ typedef enum : NSUInteger {
 
 - (void)markVisibleMessagesAsRead
 {
+    if (self.presentedViewController) {
+        OWSFail(@"%@ Not marking messages as read; another view is presented.", self.logTag);
+        return;
+    }
+    if (self.navigationController.topViewController != self) {
+        OWSFail(@"%@ Not marking messages as read; another view is pushed.", self.logTag);
+        return;
+    }
+
     [self updateLastVisibleTimestamp];
 
     uint64_t lastVisibleTimestamp = self.lastVisibleTimestamp;