static/ext/src/selection/CheckboxModel.js
function(view, initial){
        var me = this,
            checkbox = me.injectCheckbox,
            headerCt = view.headerCt;

        // Preserve behaviour of false, but not clear why that would ever be done.
        if (checkbox !== false) {
            if (checkbox == 'first') {
                checkbox = 0;
            } else if (checkbox == 'last') {
                checkbox = headerCt.getColumnCount();
            }
            Ext.suspendLayouts();
            if (view.getStore().buffered) {
                me.showHeaderCheckbox = false;
            }
            headerCt.add(checkbox,  me.getHeaderConfig());
            Ext.resumeLayouts();
        }

        if (initial !== true) {
            view.refresh();
        }
    }
function(view, initial){
        var me = this,
            checkbox = me.injectCheckbox,
            headerCt = view.headerCt;

        // Preserve behaviour of false, but not clear why that would ever be done.
        if (checkbox !== false) {
            if (checkbox == 'first') {
                checkbox = 0;
            } else if (checkbox == 'last') {
                checkbox = headerCt.getColumnCount();
            }
            Ext.suspendLayouts();
            if (view.getStore().buffered) {
                me.showHeaderCheckbox = false;
            }
            headerCt.add(checkbox,  me.getHeaderConfig());
            Ext.resumeLayouts();
        }

        if (initial !== true) {
            view.refresh();
        }
    }
Add the header checkbox to the header row
@private
@param {Boolean} initial True if we're binding for the first time.