The gradient fade under the sticky user header was absolutely positioned at
top-full with h-4/sm:h-8, so it overlapped the first rows of the assistant
content below and obscured readable text (especially for headerless messages
with pt-0). Reserve the fade as bottom padding on the sticky container and
anchor the gradient to bottom-0, so it only covers the header's own padding
box and stays purely decorative with pointer-events-none.
Fixes#2524
Serhii Dziupin
6622d8889d