消息跟随异常

[MasonXon]

当同时弹出来多个消息气泡的时候，消息列表的消息无法跟随显示最新的消息：

[RussellLuo]

@MasonXon 这个是有意设计的：

1. 如果滚动条在底部，会自动跟随新消息；
2. 如果滚动条在中间，用户可能在看中间的消息，则不自动跟随。

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This is an intentional design:

1. If the scroll bar is at the bottom, it will automatically follow new incoming messages;
2. If the scroll bar is positioned in the middle, the user is likely viewing earlier messages in the middle position, so automatic scrolling will not be triggered.