Field iframe outset margin + padding

[dndrsn]

I'd like to be able to add a focus box shadow to field(s) inside a custom field editor to match the default field behavior. Because the field editor is rendered inside an iframe the box shadow on the left and right of the field(s) will be cut off if the fields are horizonally aligned with other field inputs.

Here's an example with the proper alignment:

And an example with the field inset slightly into the iframe. The box shadow is now rendered correctly, but the field is now inset horizontally from the correct alignment with its label and the field above (it's subtle, but annoying):

If it were possible to slightly "outset" the iframe with negative margin and add padding inside the iframe to offset that negative margin the desired effect could be achieved. This could be the default behavior or an option. Any way to accomplish the desired behavior would work.