Fix rounded corner / background gap in ContextMenu

[jankeromnes]

Bug description

There is a breeze coming through this gap, and it should be fixed, ideally before the winter. 🌬️ 🥶

From #5077 (comment) :

issue: This is minor but there's a small gap on the corners due to corner radiuses. It should be barely noticable but if there's an easy fix let's also include it here. Otherwise, a follow up issue could suffice. 🏓

A	B

Steps to reproduce

1. Click on any DropDown / ContextMenu in the dashboard
2. Zoom in
3. Feelsbadman

Expected behavior

3. Feelsgoodman

Example repository

https://github.com/gitpod-io/gitpod

Anything else?

Do you have any fries? 🍟

[jankeromnes]

@gtsiolis About the good-first-issue label: I considered adding it, but then I wasn't sure, because I myself don't know how to fix this problem (I tried a few things, like not rounding the background's corners but using overflow: hidden instead, but that didn't work).

In general for good-first-issues, I like at least being able to point interested contributors in the right direction for a fix, but here I don't really know. 😬

[gtsiolis]

@jankeromnes fair point! No strong feelings either way. Removing the good-first-issue label. 🏀

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.