This tries to find a split point in the list of toplevels to maximize
the scale factor when calling `RowColToplevelLayout` twice. If it
doesn't get a better scale factor, it just uses a single row/column.
Some things don't seem quite right, but the existing layout is not
perfect, and this can help.
Without the added cross axis spacing, there's overlap, so there may be
something wrong with the requested sizes...
Previous definition assumed the cross axis of a single window would not
need more downscaling than the total main axis of all children. But that
doesn't always work out.
