Dynamic item-input-inset in Footer triggered by button


So i have a button to add an element to the list inside the footer and when it gets pressed an input appears in the footer and the button changes it’s icon to a checkmark.


If you try it you will see that the button changes position by a few px when you press it.

The thing is that i needed to add the “pull-right” class to the button (when its not in “adding mode”) because if i didnt the button would show up in left side, and when pressed, the input would take that space and move the button to the right.

I would like to be able to avoid this tiny button position change but i dont know how, im not very good with css so i hope someone can help me.