Users expect to be able to scale the uppermost edge of the asset shelf,
which actually belongs to the asset shelf header region only. So it
would only work to hide/unhide this. They expect this because they seem
like one region, but they are actually implemented as two (to have
separate layout and scrolling mostly).
This adds a region flag so that scaling a region can actually affect the
previous one instead. Something similar is already used for split
regions.
Part of #107881.
87aa34d801