I apologize for being too unspecific.
"Does not work" should mean: the page property
@Property @Persist
private String simpleProperty;
is not updated by the form, when the mixin is used.
It should be, as you can see by looking at the form's definition:
<form t:type="Form" t:zone="resultZone" t:mixins="SimpleMixin">
<t:label for="searchTerm"/>
<input t:type="TextField"
t:value="simpleProperty"
t:id="searchTerm"/>
<input t:type="submit"
t:clientId="Submit"
t:id="submitButton"
value="Submit"/>
</form>
No exceptions were thrown. There were no error messages. But when the zone is updated after form submission, only the page without mixin does update the page property.
Because both pages only differ in the mixin that is applied to the form component, I suspect that the mixin is the reason that the TextField does not update the page property. But that's just a guess, and my knowledge about T5 is still limited.
Attached a demo project, so that the behaviour can be examined.
First have a look at the SimpleMixin class. As you can see, it's one of the simplest mixins that you may think of.
Then have a look at the "ExampleWithoutMixin" page. It has the desired behaviour.
At last, try out the ExampleWithMixin page. It's the same as the page that works. The only difference is the mixin used.