The question box folded far too late because the old dock was IN FLOW
inside the scroller: folding it shrank the scroll content, so the fold
had to wait until the learner was a full dock-height past the bottom —
any closer and the browser's scrollTop clamp pinned them back to the
bottom, re-expanding the box (the oscillation the old hysteresis
comments worked around).
The dock now overlays the column bottom (absolute — a sibling of the
scroller, the use-stick-to-bottom pattern, like the scroll button), and
a spacer at the transcript's end reserves its height (DockOverlay
measures its settled block and reports upward; ChatView's dockReserve
state feeds the spacer). At-rest geometry is identical: the spacer
reserves exactly the overlay footprint.
The fold is now VISUAL ONLY: COLLAPSE_DIST 64px (was dock-height+64),
EXPAND_DIST 16px, position-based, no velocity. The reserved tail GROWS
immediately (typing, new variants) but never drops while a question is
open — a shrinking tail would reintroduce the clamp; in chat mode drops
apply (the composer typing-shrink is the old harmless behavior).
expandIntent (2s bar-click suppression) and the teach reveal gate are
unchanged.
Verified live (headless chromium, dev 3001, pending-quiz chat): teach
invitation mounts collapsed; click → expand + bottom; scroll up 200px →
folds to the bar at d=200 with the spacer held (no geometry change, no
yank); back to bottom → re-expands; spacer ≡ overlay footprint.
Note: the overlay spans the column's full width, so on platforms with
classic (non-overlay) scrollbars its gradient/background strip covers
the bottom of the scrollbar track — invisible on macOS overlay
scrollbars; flagged for master.
amosblomqvist
07415048df