It is sometimes required to add some more pixels around the element that is being cropped. Currently only the element is cropped.
Some examples where it can be useful:
https://www.mediawiki.org/wiki/File:VisualEditor_-_Edit_tab.png
https://www.mediawiki.org/wiki/File:VisualEditor_-_Section_edit_links.png
https://www.mediawiki.org/wiki/File:VisualEditor_-_Edit_tab_-_Rus.png
https://www.mediawiki.org/wiki/File:VisualEditor_-_Section_edit_links_Rus.png