Hi David
I have a button on a form that toggles a setting on and off and then redraws the tabs as needed. You may find that web client requires the additional parameters like tab text, tab position etc to work properly. (I tried taking out all the parameters except the form name and that didn't work for me even in smart client!)
The issue was a genuine bug and resolved by Johan who gave me a tweaked set of files - the change is going to be added into the next iteration of Servoy and therefore may already have been fixed. Essentially, what I was doing was correct and the smart client was behaving as expected the web client was not. I can confirm that my version is now working.
Gordon