Is it just that the viewport is centering on the button when it's re-selected? Obviously without a visual it's hard to tell; one thing of note is that all grid squares will have the exact same size, which isn't true of hbox, which may be a reason for the discrepancy in behaviour.
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What do your ATL transforms look like?
To be honest, I'm not sure this is a good use case for the controller viewport, or even a viewport at all. Given the animations you want to do, I'd probably just manually assign left/right to swap whatever card is in the center, and use ATL animations to move them to the right location. The cards wouldn't even need to be buttons; you can just have a variable track which one is selected and perform an action accordingly when the button_select action is performed.
transform cardfliphide: xzoom 1.0 ease 0.25 xzoom 0.0 transform cardflipshow: xzoom 0.0 pause 0.4 ease 0.25 xzoom 1.0 transform cardflipcombined: xzoom 0.0 # small start on show: pause 0.4 ease 0.25 xzoom 1.0 # stretch to usual size on hide: ease 0.25 xzoom 0.0 # return to small again
These are my transforms :]
I am not incredibly well-versed in ATL is the thing, so I'm not even sure where I'd begin to do something like that. I thought this was a fine use-case from that perspective. Given they're designed for menus and this is a menu, anyway.