Override that fixes a few things when calculating the min width of a combo's input field.

Combo.js

Ext.define(null, {
    override: 'Ext.form.field.ComboBox',

    /**
     * Override that fixes a few things when calculating the min width of a combo's input field.
     *
     * First, if a display field is not used but instead a displayTpl, then this override handles that
     * Also includes the 'emptyText' in case it is longer than the currently selected text
     */
    getGrowWidth: function() {
        var me    = this;
        var value = me.inputEl.dom.value;
        var field;
        var store;
        var dataLn;
        var currentLongestLength;
        var i;
        var item;
        var itemLn;

        if (me.growToLongestValue) {
            field                = me.displayField;
            store                = me.store;
            dataLn               = store.data.length;
            currentLongestLength = 0;

            for (i = 0; i < dataLn; i++) {
                itemLn = 0; // reinit to 0 for each pass
                if (me.displayField) {
                    item = me.displayTpl.apply(store.getAt(i).data);
                } else if (field) {
                    item = store.getAt(i).data[field];
                }

                if (item) {
                    itemLn = item.length;
                }

                // Compare the current item's length with the current longest length
                // and store the value.
                if (itemLn > currentLongestLength) {
                    currentLongestLength = itemLn;
                    value                = item;
                }
            }
        }

        // OVERRIDE - BEGIN
        // return value;
        return value.length < me.emptyText.length ? me.emptyText : value;
        // OVERRIDE - END
    }

});