Remove hidden overflow from site-header

[allancole]

• Prevents long sub-menus from getting truncated by the header height
• Fixes: Menus: Long menus can get cut off by content when expanded #720

Before:

After:

[kjellr]

Confirmed this works for me. 👍

[LittleBigThing]

Hi,

@kjellr and @allancole: This might be trickier than it looks. This overflow: hidden was introduced in #450 to ensure that the background image is not visible outside of the header. Removing it seems to break things in Edge and IE11.

I would have to look into this in more detail to find a solution for all browsers... any ideas?

[kjellr]

Thanks, @LittleBigThing — I'll remove the approved for merge label until we double check that.

[kjellr]

@allancole — As per @LittleBigThing's notes above, this PR still had issues in IE, so I'm not going to migrate it over to Trac. The related issue is over here now:

https://core.trac.wordpress.org/ticket/45901

Let's follow up in the new thread moving forward. Thanks!