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Instead, Tapestry creates a random-ish unique id suffix, such as "12a820cc40e" in the example; this suffix is appended to all allocated ids to ensure that they do not conflict with previously rendered ids.

Why do I sometimes get the exception "The rendered content did not include any elements that allow for the positioning of the hidden form field's element." when rendering an empty Zone?

As part of Tapestry's form processing, it must write a hidden input element with information needed when the form is submitted. Since the content of a Zone may be changed or removed, a hidden field is created just for the Zone, separate from the rest of the enclosing form.

At the same time, Tapestry wants to position the <input> field in a valid location, and HTML defines some constraints for that; an input field must appear inside a <p> or <div> element. In your empty Zone, there's no place to put the hidden element.

The solution is to add the following to the body of your Zone:

<div class="t-invisible"/>

This ensures that there's a place for the hidden input field. The "t-invisible" CSS class ensures that the <div> does not display or otherwise affect layout.

{scrollbar}