You can toggle a checkbox both by clicking the checkbox and its label. This makes them feel like they form just one clickable block. But there is a small gap between them that does not clickable. That gap is hard to hit, but when you do click on it, it is quite irritating that the checkbox doesn't react to the click.
You can test this live with the checkboxes on edit pages, or just in the demo on https://doc.wikimedia.org/oojs-ui/master/demos/?page=widgets&theme=wikimediaui&direction=ltr&platform=desktop. Note that this also applies to radio buttons, but not to CheckboxMultiselectWidget, RadioSelectWidget and related. These toggle even if you click between input and label, just like I'd expect it.
