Many people want more detailed status when you resolve a ticket. This patch will display a Custom Field on Update.html when the DefaultAction is "resolved".
If you need more generic solution to editing ticket's custom fields on reply, comment, resolve... then look at EditCustomFieldsOnUpdate.
Apply the patch from list archive.
$CustomFieldName to match the name of the Custom Field you want to display.
It won't work if, for example, you click "Reply" and change the status to resolved. This only works when the DefaultAction to Update.html is "resolved".