The previous pass measured with a 50ms timer and reported zero. A
MutationObserver plus the Layout Shift API disagreed: three log-height
shrinks per turn and two places where the reply's text went backwards.
Timer sampling had simply missed the frames.
Raw markup was flashing. A directive line arrives before the block it
opens is complete, and until then parse() has no reason to treat it as
anything but prose -- so "::task match" and "::words" rendered as visible
text for one frame and then vanished when the next line closed the block.
The streaming preview now drops :: lines outright, since blocks are not
rendered until the turn commits. It also trims the blank line that
preceded the directive, which MessageBody renders as a 9px gap: that gap
opened and closed on every block boundary.
The answered exercise no longer collapses. It was replaced by a one-line
"exercise answered" the instant Send was pressed -- around 170px out of
the log in a single frame, the largest jump in the view, and it threw away
what the learner had typed. It now stays rendered read-only, dimmed, with
the answers still legible.
Autoscroll never worked on a phone. Below 900px tutor.css sets the log's
max-height to none, so it has no internal scroll and setting scrollTop did
nothing -- a new reply just landed below the fold. It now scrolls whichever
element is actually the scroller. In the page-flow case the target is the
composer, not the bottom of the document: the word rail renders below the
chat, so following the document would scroll past the whole rail on every
turn. A reader who has scrolled up is still left alone.
Measured per turn, desktop: text regressions 2 -> 0, log-height shrinks
3 -> 0, DOM mutations 9 -> 5, CLS 0.0026 (only the word rail's own footer,
which is real content changing). Mobile: the composer stays in view and
the newest message is on screen; scrolled-up reader unmoved at y=200.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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