Correctly divide the fractions of *available* requested child-space.
This fixes issue with Trigger Page bottom attachment not showing
Frame content.
When calculating constraints the divider width/height was ignored.
When setting child allocation, the divider girth was only
subtracted *after* the first child, and iteratively subtracted
after each subsequent child widget.
This over-allocated the first child and later child widgets
were increasingly too small (factor * remaining).
882ed91f2f