Hovering z-index of a component sharing the border

[ysds]

I have been wondering about the necessity of z-index when hovering for the past few months. I think that the reason why z-index is given is related to display of a border between elements, but I do not understand exactly why it is necessary.

Related components (sharing the border)

Currently, the following components have z-index: 1 when hoverring:

• Button group (added from bfc4aaf)
• List group (added from adds hover state to the button element for background color utilities #24813)

But the following components don't have:

• Pagination
• Elements in input group (added from bfc4aaf but removed at Rewrite input group component #25020)

My points:

1. List group item issue

When hovering a list item, the border flickers slightly. It may be correct when items are links or buttons. But it is strange if it is a static list. You can see in the Contextual classes docs

2. Necessity (Clarify the design specifications)

If I may ask, could you clarify the reasons to bring it to the front when hovering? This will also be related to #25165.

Component states should obey a common z-index scale: default (initial), hover (1), active (2), focus (3)

3. Consistency

If the hovering z-index is the correct specification, please consider adding it to pagination and input group as well.

Thanks.

[mdo]

I've used overlapping borders to prevent us from having to outright remove a border. This creates a more seamless component and (I think?) reduces repaints as you modify the sub-components. By default, sibling elements that follow a particular element will be "above" those on the z-index scale (e.g., the 2nd is above the 1st, and the 3rd is above the second). So, we need to modify those on hover to allow say a darker border or background to come to the forefront. It's largely a visual design necessity, and I agree it needs uniform application.

[mdo]

Pagination has the z-index on hover, and input group has it in general for buttons and add-ons from the looks of it. Let me know if there are any specific lines that need updating that I missed, though!