You can stack banners horizontally!

Real version: 21w05b

What I expected to happen was...:
Spam place banners around a wall.

What actually happened was...:
I stacked the banner on its side and now there's a floating banner.

Steps to Reproduce:
1. Place a banner on a wall.
2. Place a banner on the banner you just placed (top half).
3. You have stacked a banner horizontally.