fix scroll hanging when scroll all the way up

ts/components/session/conversation/SessionConversation.tsx

@@ -188,8 +188,7 @@ export class SessionConversation extends React.Component<Props, State> {
       div?.addEventListener('dragleave', this.handleDragOut);
       div?.addEventListener('dragover', this.handleDrag);
       div?.addEventListener('drop', this.handleDrop);
-      },
-    100);
+    }, 100);
   }
 
   // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -634,8 +633,7 @@ export class SessionConversation extends React.Component<Props, State> {
     const conversationModel = window.ConversationController.getOrThrow(
       conversationKey
     );
-    const selectedMessages = messages.filter(
-      message =>
+    const selectedMessages = messages.filter(message =>
       this.state.selectedMessages.find(
         selectedMessage => selectedMessage === message.id
       )

ts/components/session/conversation/SessionConversationMessagesList.tsx

@@ -324,10 +324,11 @@ export class SessionConversationMessagesList extends React.Component<
       const numMessages =
         this.props.messages.length +
         Constants.CONVERSATION.DEFAULT_MESSAGE_FETCH_COUNT;
-      const previousTopMessage = messages[messages.length - 1]?.id;
+      const oldLen = messages.length;
+      const previousTopMessage = messages[oldLen - 1]?.id;
 
       await this.props.getMessages(numMessages);
-      if (previousTopMessage) {
+      if (previousTopMessage && oldLen !== messages.length) {
         this.scrollToMessage(previousTopMessage);
       }
     }