position:sticky inside accordion

Created by: PatrickRose

• Operating system and version: Linux
• Browser and version: Chrome 73 (but reproducible on all browsers)
• Reduced test case with fix

In #27133 to remove the double border at the bottom, cards inside an accordion got overflow: hidden done. This means that position: sticky doesn't work (my guess is that because the browser is hiding things that are overflowing, sticky doesn't trigger?).

The codepen attached replaces that with a removal of the border - since that's what we're actually trying to achieve but I'm not 100% sure that's the right solution.