[Bug Report] When the tree is dragged with a scroll bar, the reference line position is calculated incorrectly

### Element UI version
2.15.0

### OS/Browsers version
Mac11.1/Chrome88

### Vue version
2.6.11

### Reproduction Link
https://codepen.io/joel-zh/pen/qBaevZa

### Steps to reproduce
After the vertical scroll bar scrolls down, drag the node.

### What is Expected?
Whether the vertical scroll bar is scrolling or not, it does not affect the dragging reference line.

### What is actually happening?
When the scroll bar is scrolled down, drag the reference line to offset upward, the more you scroll, the greater the offset.