The distribute_natural_allocation() function was copied over from Gtk to
Clutter 11 years ago with commit e636a0bbce2b24fdb4e99dcf2d4385eb25d0247a.
Gtk only supports integers sizes in its layout machinery, while Clutter
does everything using floats.
Since this function sets the minimum_size (the size we allocate the
children in the end) to an integer, this means we're implicitly
typecasting floats to integers here, effectively floor()'ing all sizes
that we allocate the box children.
A bug this caused in gnome-shell was that a scrollView (like the one in
the endSessionDialog) was showing scrollbars even though the content
perfectly fit inside the view: Say the content and its scrollView parent
request a size of 63.9 px, but get allocated a size of 63 px by a box
layout. Now the scrollView notices that its allocated size is smaller
than the requested size and thus shows a scrollbar.
So fix that and use floats in distribute_natural_allocation() instead of
integers, as we do everywhere else in the layout machinery.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2284>
abecf557bf