So did a big "duh" and double-clicked the nifty "yield" sign in the
lower left chrome and I see there is a JavaScript error on the page.
Something like this:

line 3187
char 29
Error: 'undefined' is null or not an object
Code: 0

Looking at my (Rails-generated) "all.js" in Firebug (in Firefox), line
3187 appears to be:

3184 attrPresence: function(nodes, root, attr) {
3185 var results = [];
3186 for (var i = 0, node; node = nodes[i]; i++)
3187 if (Element.hasAttribute(node, attr)) results.push(node);
3188 return results;
3189 }

which is part of the Selector class (in Prototype).