[iOS] Grid in UserSelectView gets cut off with more than one user

[samglt]

The grid of user icons in the UserSelectView gets cut off and doesn't scroll when more than two icons are shown. This looks like it was introduced in #1328.

I think the issue is that something in the UserGridButton is using trackingSize() while the VGrid's .scrollIfLargerThanContainer() also uses trackingSize() to determine its frame size. Maybe the GeometryPreferenceKey gets set to multiple values within the same tree at the same time and breaks the scrollIfLargerContainer.

I can't untangle exactly which change would have caused this to happen though since the previous commit also has uses of trackingSize inside the UserGridButton.