Popover tabindex when reference is an input

[aphofstede]

Element UI version

2.4.0

OS/Browsers version

Chrome 66 macOS

Vue version

2.5.16

Reproduction Link

https://jsfiddle.net/mmx38qxw/2021/

Steps to reproduce

Create a Popover with an input as a "reference" element. The form field now needs two tab presses to get focus. (Caused by the tabindex being set to '0' hard-coded for the reference field)

Work-around:

    <popover ref="elPopover" ...>
    ...
    mounted() {
        this.$refs.elPopover.referenceElm.setAttribute('tabindex', -1)
    },

Related: #9038

What is Expected?

The native reference element (input, textarea, etc.) gets the tabindex

What is actually happening?

The reference element's container gets a tabindex

[ghost]

Yes, it is hardcoded. This should be optional.

[ziyoung]

ref #11875