Catalog client scripts - puzzling behavior onsubmit with onchange

I happened to notice an issue which was particularly apparent in Service Portal view, and wasn't made easier by the fact that in Service Portal, test alert messages appear to get caught in a loop until you click out of the tab and back in...
I haven't got to the bottom of it yet, but I did notice in my scenario where the following was happening:

-onchange runs on a checkbox which sets a number of fields, and resets fields to blank if unchecked
-onsubmit vets whether the checkbox is ticked

that the onsubmit catalog client script was calling the onchange code, even though the checkbox was not being set in the onsubmit, merely it's value of true or false was being obtained

the problem was that the onchange was running and messing up the related fields...so,
I seem to have got round it by using the condition in the onchange:

if (newValue=='false'){

     //reset fields

     //...

     return;

}

Still investigating a better solution...and trying to figure out if there is a way to determine in an onchange script that the form is being submitted, to arrive at a solution like this:

if (<form_is_submitting>=='true'){

     //reset fields

     //...

     return;

}