Instead of using len(visible_after)-1 (which points to the last visible
message and gets pushed behind the render window as more turns accumulate),
find the last [CONTEXT COMPACTION] marker in s.messages and compute the
anchor from visible messages before it.
This keeps the compression reference card at the correct boundary even
after 50+ subsequent turns have scrolled the render window past the old
anchor position.
Fixes a bug where the assistant's output message appeared to disappear
after automatic context compression because the reference card was placed
at the wrong position.
fxd-jason
90dfbf2f2d