fix: Calendar card "..." action menu overlapping behind adjacent cards

[Pinjasaur]

Summary

This PR unsets some z-index values that Full Calendar sets to prevent the "..." action menu from being overlapped by adjacent events in Calendar view.

I went through a couple iterations to get to this solution, but it ended up being the last "hacky" (subjectively) even though it is overriding CSS from a vendored dependency. For some context, other "solutions" I tried:

1. Reverse the event render order in Full Calendar and then use flexbox to reverse it. This didn't work with multi-day events along with the "show more" calendar widget.
2. Use a SCSS @for loop to set descending :nth-child z-index values e.g. https://stackoverflow.com/a/36609688

Before:

After:

Ticket Link

closes #3425

[asaadmahmood]

@Pinjasaur Wouldn't a better solution be just to add a class to the card who's menu is open, and then just increase the z-index on it?

I think we can then also use that functionality to make sure that the menu remains even if you hover outside of that card, and only remove that class on a blur event or on a click outside.

[Pinjasaur]

@Pinjasaur Wouldn't a better solution be just to add a class to the card who's menu is open, and then just increase the z-index on it?

I think we can then also use that functionality to make sure that the menu remains even if you hover outside of that card, and only remove that class on a blur event or on a click outside.

@asaadmahmood We don't have a lot of control over how the Calendar view is rendered since it's mostly done by the Full Calendar library component. I went the unset route for the existing z-index values because—from how I understand it—when they were set a CSS stacking context was being created so the adjacent elements would naturally stack over the preceding siblings.

I think your idea is worth exploring, but perhaps as part of #1007 since it might also solve that same issue that bug is tracking. Let me know your thoughts—we're looking to get this for 7.2 or otherwise revert the functionality of the "..." menu (on Calendar view cards) for the release since it's broken as-is. #3425 (comment)

[asaadmahmood]

LGTM for now.

[mattermod]

Cherry pick is scheduled.