From WEB Core version 2.6.0.0 the form which is open with popup := true behaves strange:
The popup form is not centered on the screen
If the element width is large the form doesn't automatically grow down
I can't see the items from 0 to 3
In version 2.5.4.0 it behaved these two points are ok.
The problem is that the TMS Web Core is not backwards compatible I have to fix these kind of problems.
It respects the defined form height, so your list is larger than your form, so set the form height large enough (as you set HeightStyle = ssAuto, so it takes more height than the form height)
Why the ElementClassName and ElementPopupClassName doesn't work. When I set some my class name like below for the popup form the final rendering has only class="card" without our class. It was working in 2.5.4.0