You need to add a div clear at the end of your widget code. This allows browsers to properly calculate the height of the widget div since all your elements are floating.
I added this line to wp125.php around line 328 right before the function close curly brace:
echo '<div style="clear:both;"></div>';
Also, you should add more hooks and filters into your plugin so that other developers can customize the output of your code. Its nearly impossible to do anything custom with your plugin.