Is the data in the text box to be validated only when leaving the box, or during typing (I'm not assuming browser or native application here)? As qhoxie mentio

Is the data in the text box to be validated only when leaving the box, or during typing (I'm not assuming browser or native application here)? As qhoxie mentioned, not simply discarding but warning the user or saving a cache if the text box contains more than a simple few characters. When/if the user returns to the page/panel/dialog, scope is immediately brought back to the "unfinished" box and validation can continue.