Bug 225590 changed flex behavior in such a way we can no longer use `height: 100%` on all the child sections of the navigation sidebar in the Sources Tab to progressively size each section based on available space. Bug 227597 removed the `height: 100%` to allow the various sections to always be visible again, but the resize behavior is not an exact match for before the regression point, only better. Once bug 227604 is resolved, we should be able to reliably use flex-basis to achieve the desired behavior.
<rdar://problem/80055034>