流式传输使用updateMsg更新消息时会强制滚动到最下方，若此时手动向上滚动浏览消息会出现冲突造成页面抖动

**What problem does this feature solve? (这个功能解决了什么问题)**
版本："@chatui/core": "^2.4.2"
每次在距离底部两个当前窗口高度内更新消息都会强制滚动，无法正常浏览消息

**Describe the solution you'd like (描述您想要的解决方案)**
用户可以自定义范围触发滚动比如距离底部0.2个窗口高度滚动，或者距离底部一定距离内滚动如距离底部100px内触发滚动

**What does the proposed API look like? (你期望的 API 是怎样的)**
暂时没深入了解相关API，不做评论

Start by reproducing the issue with updateMsg while the user scrolls upward, then trace the existing scroll-trigger behavior in the ChatUI source. Done means users can configure a bottom-distance threshold, such as 0.2 viewport heights or 100px, without scroll conflicts or page jitter.

Written by the indexing model from the issue text.