@mvorisek Why do we need this BC-break? It is very common to run set and just pass any model field to fill in template variables. The BC-break leads to an error because each and every set variable has to be first converted to string.

Use Case:
You have a HTML invoice template and want to set the integer order number.

Before:
$template->set('invoice_no', $model->get('invoice_no'))

Now:
$template->set('invoice_no', (string) $model->get('invoice_no'))

What is the benefit justifying this BC-break?

Please read the PR description and let's continue the discussion in a private chat, as the real impact should be minimal with the best atk4/ui usage.

Why is PHP-standard typecasting not allowed here where model field is an integer?

\Atk4\Ui\Header::addTo($app, [$model->get('stock')]); 

Simple code, simple PHP typecasting. Do we really want to enforce everyone to code like this?

\Atk4\Ui\Header::addTo($app, [(string) $model->get('stock')]);