This patch fixes the usability issue where the user has unsaved changes on a page, and accidentally clicks a link or the back button and loses all of there changes. This patch uses the onBeforeUnload event to warn the user that they are about to lose their work, and gives them a chance to cancel. It currently just works on the blocks admin page, but would be easy to extend to other pages. onBeforeUnload works on Firefox, IE and Safari, but not Opera.
Note, this patch is much, much simpler that what I originally proposed over at: http://drupal.org/node/193311 (which remains a cool set of functionality, but is overkill to solve this particular problem). This patch is about 5 lines of code.