Setting Select to to 'disabled' does not disable the element. The user can still click on the text of Select2 and the options box opens up. Here is a disabled control that was opened by clicking on the text and not the down-arrow button.
Here is my code:
<?= $form->field($model, 'billing_currency_id')->widget(Select2::className(), [
'data' => BillingCurrency::listIdCodes('','',true),
'disabled' => true,
'options' => ['disabled' => true,],
'pluginOptions'=>[
'allowClear'=>false,
'dropdownAutoWidth'=>true,
'disabled' => true,
], ]); ?>
Clicking on the down-arrow button keeps the control closed, but clicking on the text area of the control opens the options box.
UPDATE Found my own mistake - see answer below.