The active composer seat now keeps one stacking rank (z-index 9) in every
state, a direct compositing reason (will-change: transform), the scroll
floor without relying on a View to fill the column (margin-top: auto), and
no scroll-anchoring role. Composer tooltips portal to the body because the
seat is the containing block for fixed descendants. The width handle keeps
z-index 0 while dragging.
The docked statistics row reserves one reading's height before its
projections arrive, and the header without a Session keeps the blank
Session's 40px row on every platform so both Hero states center the input
card at the same height.
Fixes#7590
imccyu