Laurent,
IMHO the next method down in the documentation ensureElementVisible() is the better method to use for the folder page problem
https://4js.com/online_documentation/fjs-fgl-2.30.01-manual-html/User/ClassForm.html#ensureElementVisibleIn your form, name the page elements e.g.
PAGE pageonename (TEXT="Page One")
... and then use the new ensureElementVisible() method passing the name you have assigned to the page e.g.
DEFINE f ui.Form
CALL f.ensureElementVisible("pageonename")
... that way the code is referring to the folder page you want on top, not an arbitrary field on the page as with ensureFieldVisible.
Reuben