[Bug Report] Color-picker does not create the popup next to the rendered color picker element

### Element UI version
2.15.7

### OS/Browsers version
Windows 10

### Vue version
2.6.14

### Reproduction Link
https://codesandbox.io/s/broken-flower-veznh

### Steps to reproduce
1. Click on the launch modal button
2. Try to select the input that is created by the color picker to enter hex

I have added an input to the modal to show that it does work when the element is rendered within the modal

### What is Expected?
the color-picker should not be injected so far away from where the component is rendered ideally it would be next to the picker, otherwise, it will always have issues like this.
alternatively given the option to specify which element to come after/ be attached to.

### What is actually happening?
you cannot select the input to type a hex value