Steps to reproduce:

• Add a Ui\Dropdown view and setModel for any model with a usual bigint idField
• Configure Dropdown->onChange to handle selected item
• Select an entry of the dropdown

Sample code:

    $agentdropdown = \Atk4\Ui\Dropdown::addTo($app, ['Select agent', 'dropdownOptions' => ['on' => 'hover']]);
    $agentdropdown->setModel($agentmodel);

    $agentdropdown->onChange(static function ($itemId) {
        return "do something with ".$itemId;
    });

Typecasting error TypeError: Atk4\Ui\Dropdown::Atk4\Ui\{closure}(): Argument #2 ($value) must be of type string, int given, called in vendor/atk4/ui/src/JsCallback.php on line 90

Solution:

The itemId here is forced to be string, however the idField is typecasted as int. This has to be adapted.