Bad location of dropdown menus when the trigger button is close to the end of the screen

See:

Steps to reproduce:

• set the language to "fr"
• remove the right column panel
• click on "plus d'actions"

We can still scroll to see the end of the menus, but it's very not friendly. It's even worse on mobiles.

I have reported this to bootstrap but they won't fix it. However, I have a fix.

Note: I have already fixed this in 7.2M3, but I can improve it.