The previous approach involved calculating the width of each element
independently, and deriving the `x` coordinate of each element by
summing the previous widths. However, if the widths are rounded, summing
them leads to accumulated rounding errors.
This change effectively takes the alternate approach of calculating the
`x` coordinate of each element independently, and then deriving the
widths from the differences between adjacent values of `x`.
As a bonus, this change corrects a couple issues in the
`padding_with_fill` test.
3622895f09
