FullCalendar (Filemaker webviewer) has lost interactivity when rendered, I think it is to do with perform javascript in web viewer step

https://stackoverflow.com/questions/70785249/fullcalendar-filemaker-webviewer-has-lost-interactivity-when-rendered-i-think?noredirect=1#comment125146163_70785249

I have created this post and rather than write it all out again I thought it is easier to link to it. I have not had much luck so far. Could anyone take a look and help me out. I'm going mad. 

It is to do with a fullcalendar project I am working on. I have used the frudens file as a template, I have heavily modified it. In the single file that I have modified it is working beautifully, however, I now need to integrate it into a current file. I have copied all of the scripts, custom functions layout objects etc in the right order, checked them all over, which has taken hours, but the main issue is that the calendar does not have the interactivity. It is a resource timeline calendar. When I render it, the nav buttons stop working. Also when I edit an event, the dragging, resizing or adding a new event doesn't update on the UI, it edits the event in the table, and when I leave the layout and then go back to it, it shows the updated event, but as soon as I trigger the rendering script it stops working again. 

I think it is something to do with he perform javascript in webviewer script step. This seems to be where it breaks. Image in link. 

I am thinking there must be a setting somewhere that is preventing it from updating. I have set the webviewer with the correct settings.