When I click on Preview in the WordPress editor, Fluid makes a GET request to the web server. It still uses the action address specified in the #post form (post.php), just not the POST method. But none of the form values are included, so I end up staring at a listing of all posts instead of the preview I wanted.
Maybe it is getting confused by the fact that the #post-preview “button” is actually an HTML link (an a element)? There is also some JavaScript that attaches to the click event of the link (see the doPreview() function).