OneToOne Wagtail InlinePanel

inline_panel.js

// stuff/templates/wagtailadmin/edit_handlers/inline_panel.js

(function() {
    var opts = {
        formsetPrefix: "id_{{ self.formset.prefix }}",
        emptyChildFormPrefix: "{{ self.empty_child.form.prefix }}",
        canOrder: {% if can_order %}true{% else %}false{% endif %},
        maxForms: {{ self.formset.max_num }}
    };
    var panel = InlinePanel(opts);

    {% for child in self.children %}
        panel.initChildControls("{{ child.form.prefix }}");
    {% endfor %}
    panel.updateAddButtonState = function() {
        if (opts.maxForms) {
            var forms = panel.formsUl.children('li:visible');
            var addButton = $('#' + opts.formsetPrefix + '-ADD');

            if (forms.length >= opts.maxForms) {
                addButton.hide();
            } else {
                addButton.show();
            }
        }
    };
    panel.setHasContent();
    panel.updateMoveButtonDisabledStates();
    panel.updateAddButtonState();
})();

models.py

# stuff/models.py

class SomeDocument(models.Model):
    document = models.ForeignKey(
            'wagtaildocs.Document',
            null=True,
            blank=True,
            on_delete=models.SET_NULL,
            related_name='+'
        )
    page = ParentalKey(
        'stuff.SomePage',
        related_name='my_one_document',
        unique=True
    )
    

class SomePage(Page):    
    body = RichTextField(blank=True)

    content_panels = [        
        FieldPanel('body'),
        InlinePanel('my_one_document', label="Document"),
    ]

There no longer seems to be any need to use javascript to hide the disabled + button. You can use some simple CSS, instead:

.classname-of-inline-panel:not(.empty) p.add {
  display: none;
}

Since it only hides p.add when the panel isn't empty, the + button is displayed properly when the panel is collapsed.