It was needed only for sticky to work on XUL and it's not needed anymore.
The way splitters work on modern flexbox is that they take the min-width
property of the element they're resizing (without looking at descendants).
This is done for performance reasons (flex item min-width is more expensive to
compute on isolation than XUL flex was). Removing the wrapper makes the
min-width be on the right element anyways.
Differential Revision: https://phabricator.services.mozilla.com/D160722
Emilio Cobos Álvarez