Min-height setting always centered
### Bug report
#### Expected behavior:
![](https://github.com/DWilliames/paddy-sketch-plugin/raw/master/.images/GIFs/padding-min-height.gif)
This is readme demo. I really want this function.

#### Actual behavior:
![](https://user-images.githubusercontent.com/39715793/42027699-e9c2deea-7afc-11e8-9433-8315e1d5f871.gif)

But I name the background layer "Rectangle [20;h>=250]" ,why the background layer  is always vertical centered? not like Paddy Readme's demo.


