Strange JavaScript collapse behaviour in VS2012

Strange JavaScript collapse behaviour in VS2012

Visual Studio 2012 offers me the possibility to collapse this function
(see the little "-" icon on the left).

Sometimes he arbitrarely decide I can't do that.

Can someone explain why and how to fix it? I want to collapse every
function in my code: can I do this? How?
It looks like placing $.get, $('selector') or $.each calls in my code is
causing the issue but, since it mostly looks like a random behaviour, I
can't tell for sure.
Here's some random piece of code which is causing the issue:
function ComputeHigherZIndex() {
var highest = null;
$('#@(mdi.ContainerDivId)').find('.dynawindow').each(function () {
var current = $(this).css("z-index");
if (highest == null || highest < current) {
highest = current;
}
});
return highest;
}