Views "Add fields" dialog: top/bottom borders missing from the footer divs (.views-add-form-selected and .ui-dialog-buttonpane)

[klonos]

I noticed this while trying to figure out the problem behind #6395. Here's a couple of before/after screenshots:

• w/o any field selected in the dialog:
• field selected:

The respective bottom border exists in the "header" div of the dialog:

[klonos]

PR here: https://github.com/backdrop/backdrop/pull/4752