Do not extend widget-field?
Another small detail to make the customize interface less heavy:
(set-face-attribute 'widget-field nil :box (face-attribute 'eww-form-text :box) :extend nil) (set-face-attribute 'widget-field nil :box nil :extend nil)
:box attribute does not work well for fields which span across multiple lines. Multiline fields also don't look that good with extend, so I am not sure about this change. Emacs lacks a bit of styling capabilities to make these widgets look nice.