[Bug Report] Table Tree Data and Lazy Loading, Pages without Scrollbars and unable to scroll

[Monroe123]

Element UI version

2.8.2

OS/Browsers version

Chrome

Vue version

2.6.10

Reproduction Link

https://codepen.io/pen

Steps to reproduce

提示：复现链接参考 http://jsrun.net/VWyKp/edit

步骤： 点击父节点，异步加载子节点，子节点高度超过table高度，没有出现滚动条，除非浏览器窗口变化，会引起table重新绘制

What is Expected?

右侧出现滚动条，且正常滚动

What is actually happening?

右侧没有滚动条，且无法滚动

[element-bot]

Translation of this issue:

Element UI version

2.8.2

OS/Browsers version

Chrome

Vue version

2.6.10

Reproduction Link

Https://codepen.io/pen

Steps to reproduce

Tip: Refer to http://jsrun.net/VWyKp/edit for a recurrence link

Step: Click on the parent node, load the child node asynchronously, the child node height is higher than the table height, there is no scroll bar, unless the browser window changes, the table will be redrawn

What is Expected?

A scroll bar appears on the right side and scrolls normally.

What is actually happening?

There is no scrollbar on the right and it cannot scroll