Comment 3 for bug 1511325

Hmm...
The decision is a little bit confusing.
 > gtk_widget_get_margin_right(left) has been deprecated since version 3.12 and should not be used in newly-written code. Use gtk_widget_get_margin_end() instead.

I'm not quite sure why is code reverted to xxx-left & xxx-right if it's deprecated in the required 3.12.