ajax.js insert command sometimes wraps content in a div, potentially producing invalid HTML and other bugs

Problem/Motivation

Prior to #39, Drupal.AjaxCommands.prototype.insert() always added a DIV wrapper to the content returned by the server. Since #39, it only does that if the content isn't already contained in a single root element. The current HEAD code is:

      // For legacy reasons, the effects processing code assumes that new_content
      // consists of a single top-level element. Also, it has not been
      // sufficiently tested whether attachBehaviors() can be successfully called
      // with a context object that includes top-level text nodes. However, to
      // give developers full control of the HTML appearing in the page, and to
      // enable Ajax content to be inserted in places where DIV elements are not
      // allowed (e.g., within TABLE, TR, and SPAN parents), we check if the new
      // content satisfies the requirement of a single top-level element, and
      // only use the container DIV created above when it doesn't. For more
      // information, please see http://drupal.org/node/736066.
      if (new_content.length !== 1 || new_content.get(0).nodeType !== 1) {
        new_content = new_content_wrapped;
      }

There are multiple conditions where even the more limited DIV wrapping creates problems (see comments #41-#48).

Proposed resolution

Figure out if we can improve ajax.js to never require the DIV wrapper. Specifically:

// For legacy reasons, the effects processing code assumes that new_content
// consists of a single top-level element.

Can we fix that?

// Also, it has not been
// sufficiently tested whether attachBehaviors() can be successfully called
// with a context object that includes top-level text nodes.

Can we test/fix that?

Remaining tasks

User interface changes

API changes

Original report by @peterpoe

As a workaround for a Safari bug, ajax.js wraps new content inside an empty div after a successful ajax request:

    // Manually insert HTML into the jQuery object, using $() directly crashes
    // Safari with long string lengths. http://dev.jquery.com/ticket/3178
    var new_content = $('<div></div>').html(response.data);

The div makes inserting content not at block level (like in table rows and spans), difficult to achieve. I have published a workaround for Drupal 6 here: http://drupal.org/node/472328

Since the corresponding bug has been solved in Safari since 2009-03-19, I think that both Safari 3.2.3 and 4.x won't crash anymore with long strings. Given that Safari 3 has is used by 0.3% of web users, my opinion is that the limitations posed by the workaround are now worse than the problem solved.

Attached patch changes the behavior to:

    var new_content = $(response.data);