If a scrolled view has an automatic policy for a scrollbar, it can appear and disappear at any time, according to the content.
Showing the scrollbar has the effect to change the allocation of the scrolled part of the view; this is especially noticeable for vertically scrolled views with a centered layout (like our search results), since the scrollbar will make everything shift horizontally.
One way to fix this generically is to always shrink the scrolled content allocation as if the scrollbar was visible, even if it's not.
Not sure we should do this unconditionally, so I made it a property that defaults to FALSE.