Percent heights for children of column-flow flexbox items yield counterintuitive results

We're trying to use a column-flow flexbox container to stack items in a
layout. Some of the flexbox items have fixed heights, but one of them is
allowed to stretch. Inside the flexible item we place an element with 100%
height. Intuitively, we thought this would cause the 100%-height child to
match the height of its parent element as it flexes. Unfortunately, the
spec mandates that percent sizes are only computed relative to the first
ancestor with a "definite" height. In a world without flexbox, this makes
perfect sense. But unfortunately, this rule makes vertical flexbox layouts
a lot less useful. Would it be reasonable to extend the definition of
percentage heights to allow them to be based on the "overridden" heights of
flexbox items? Thanks.

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