I investigate deeper and the problem seems to be a little more complex.

In the onfocus I activate the following code in order to have the sequence number of the element :

var elementName = getElementName(element);
for (var i = 0; i < myPage.length; i++) {
if (getElementName(myPage.elements[i]) == elementName) {
id = i;

I found that when the checkbox goes from unchecked to checked the code fails and doesn’t find the sequence number, when it goes from checked to unchecked it works and finds the correct sequence number.

Could You explain me why ?

I also listed all elements and, effectively, when the checkbox is non checked it doesn’t esist in list of the element of the pages.