The composer paints its draft in two layers — the textarea owns the value,
the selection and the caret, the backdrop paints every visible glyph — and
they had one scroll offset each, kept equal by a `scroll` listener. That
holds at rest and not in motion: a wheel gesture scrolls the textarea on the
compositor, the listener runs afterwards, and for those frames the caret sits
at the new offset with the words at the old one. Measured on a harness of the
same geometry, a 200px offset change separates caret from glyphs by ~200px
(chromium 203, firefox 202, WebKit 203) until a later frame — the caret
flying out of its own text when a user swipes a long draft quickly.
Both layers now ride one scrollport: `[data-input-scroll]` carries the
14-line cap, the auto-grow stack inside it is as tall as the whole draft, and
the textarea holds no scrollable overflow of its own. The browser applies one
offset to both layers in the same frame, so the coupling is structural rather
than maintained. The backdrop's trailing-line sentinel and the cross-engine
wrap-width premise go with the mirror: the layers now share a containing
block, which closes the WebKit 768-against-776 gap by construction.
The catalog's context window is now a text field that reads a decimal K or M
suffix — 1M is 1000K, matching how model capacities are quoted — and stores
the plain token count, so settings.yaml and the adapter are unchanged.
A stored count reads back in the shortest form that round-trips: 1000000 as
1M, 256000 as 256K, and 131072 written out, because it is not a whole number
of thousands. The field holds the typed text while its row has focus, since
re-deriving it from the parsed count on every keystroke would rewrite 1000
to 1K mid-word; text that does not parse stays on screen so the save-time
rejection names a row the user can still see and correct.
Review flagged that "equal by construction" rested on an engine behaviour I
had not measured: `scrollbar-gutter: stable` only equalizes the layers if the
engine applies it to `overflow: hidden` the way it does to `overflow-y: auto`.
Measured it on the running app across the three engines Playwright ships,
and the property does not hold up.
engine .input / .backdrop / .mirror wrap width
chromium 776 / 776 / 776 (768 / 768 / 768 with the declaration)
firefox 776 / 776 / 776 (unchanged by it — overlay scrollbar)
WebKit 768 / 776 / 776 (unchanged by it)
WebKit reserves for `overflow-y: auto` and not for `overflow: hidden`, so the
declaration left .input at 768 against 776 — exactly the gap it was meant to
close — on the one engine where that gap is observable at all, while costing
every chromium user 8px of text column unconditionally. Reverted: the
composer's metrics are now the same as before this PR.
The WebKit gap predates this change and is not closed here. It is recorded in
the Agent Note with the numbers, and the browser scenario asserts the equality
on the lane's engine so a regression into that state fails loudly. The mirror
is unaffected on WebKit for the drafts measured — the extents still agree — but
a draft whose wrapping turns on those 8px would clamp it.
The review's monotonicity concern resolves the same way: the declaration was
never worse than master, because WebKit already measured 768 against 776
without it. It simply was not better.
Also from this round: the wrap-width assertion now covers .mirror as well as
the two glyph layers — it is the height authority, so a mirror alone wrapping
wider would measure the box short and clip content below the 14-line cap with
every other assertion green. Plus `renderGeometry`'s missing `@param
trailingNewline`, and both e2e tsconfig lists restored to alphabetical order.
Second review round escalated the wrap-width divergence from a separate
concern to a defect in this fix's own premise, and it is right.
Only .input scrolls, so only .input loses content width to a scrollbar that
consumes layout space — what Windows and Firefox draw, and what the theme's
global `::-webkit-scrollbar` width makes chromium treat as occupying space.
A narrower .input wraps a long soft-wrapped draft onto more lines, so it
grows taller, its scroll maximum exceeds the backdrop's, and the mirrored
offset clamps below the caret. That is the same failure the trailing-line
sentinel fixes, in the same direction, so deferring it would have shipped a
fix that does not hold where users run a classic scrollbar.
My first attempt to reproduce it found nothing and was wrong: the probe
content was not wrap-sensitive. With varied-length words the effect is
plain — the same draft laid out at 8px-apart widths differs by 2 to 5
lines, while at equal widths a textarea and a div agree exactly.
The three layers now reserve the gutter together, in the shared metrics
block that already exists to keep them symmetric. `overflow: hidden` is
still a scroll container, so the non-scrolling layers honour it: 8px is
reserved on each, measured. The cost is a text column 8px narrower on every
platform, which is the price of one geometry rather than a per-platform one.
The browser scenario asserts the premise directly — equal wrap widths, and a
reserved band greater than zero on each layer. The band is what stops the
assertion being vacuous: the widths would also match with no reservation at
all on this engine's overlay scrollbar, and it is the reservation, not the
match, that carries the guarantee to a platform whose scrollbar takes real
width. Removing the declaration fails it with `expected 0 to be greater
than 0`, and fails the golden with it.
Also from the same round, three comment corrections: the e2e file header no
longer describes the deleted layout effect, the measurement guard no longer
claims the backdrop holds exactly one text node (the sentinel makes a
second), and the sentinel comment now carries the one-sidedness argument
that also settles the ghost hint — the mirror only fails when the backdrop
is SHORTER, and the hint can only add content, never remove a line box.
