I think that I understand what you're trying to say... but I'd like to be more clear just to be certain.
If I were to throw together a quick form with two Lookup Controls, each lookup being inside of a different Panel, the Editor version looks like:
(Note: The Green and Orange borders are the borders of the different Panels)
Straight Forward!
If I generate a preview of the Form or Load it, I get:
If I were to interact with the 'Store Sections':
Alright! So, are you saying that what is happening isn't like this at all? That when you initially load your form, the size of the 'B' Lookup (which in my example is the 'Section Contents' Lookup w/ Panel) is sized to contain everything that could be placed there even though no selection has been made using the Control that it's Filtering on ('A' Lookup in your case, and 'Store Sections' Lookup in the above example)?
To cover the basics, is the Panel's 'Resize at Runtime' enabled and set to 'Yes'?
All that being said, if it isn't something like that (basic), then it might just be the fact that I'm running SP2016 and one Nintex Forms release back from current, and I know that they have added some new stuff for Form Height management which may or may not exist in 2013. No matter, if you can show me how you're getting the height difference between your initial Form State and the changed State, it should be rather simple to just manually resize the parent Panel to whatever you want.
In the code example you give, you reference vHeight, but there is no mention as to where that variable lives or how it gets defined. If it comes down to coding a solution, could you provide more of your code so that we can just copy / pasta / test?
Ty
UGH...Over engineered. Once I stripped out my javascript for filtering and pointed the list control A for filtering it's working as you pointed out. Gosh, Thanks for your time on this, owe ya one.
Been there done that (rather recently actually... Repeating Section Rows Resizing With Hidden Controls). After a long run of things not being simple, it's easy to get locked into the idea that nothing is simple, ha!
At least in this instance, it was, and that's always a relief (and also way less to keep up with / manage!)
Good luck and safe travels!