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.
Review caught a real divergence the earlier measurements missed: mirroring
an offset is only correct while both layers can reach it, and for a draft
ending in a newline the backdrop could not.
A textarea reserves a line box for the caret after a final newline.
`white-space: pre-wrap` collapses a text node's trailing newline and
generates none. So a draft ending in a newline made the backdrop exactly one
line shorter than the textarea — measured 628 against 652 — and the mirrored
assignment clamped, leaving the glyphs one line behind the caret at the very
bottom of the draft.
The backdrop now carries the same trailing-line sentinel the mirror div has
carried all along: its content is the decoration walk plus one newline. The
same pre-wrap collapse absorbs it when the draft does not end in a newline,
so it costs no height in the ordinary case, and it supplies the missing line
box when it does. Verified in isolation first: a bare pre-wrap div measures
180/180/198 against a textarea's 180/198/216 for zero, one and two trailing
newlines, and 180/198/216 with the sentinel.
Coverage for the shape that exposed it: the browser scenario asserts the two
extents are equal before asserting the glyphs reach the end, observing each
layer's maximum by asking for an impossible offset and reading back the
clamp rather than computing it from scrollHeight, and the golden records the
relation. The unit spec pins the backdrop's text as the draft plus exactly
one newline. Removing the sentinel fails both, the e2e with the same 628
against 652.
The scrollbar-gutter half of the same review point does not reproduce here:
both layers measure clientWidth 776 against a border box of 776 while the
draft overflows, so this engine's textarea scrollbar is an overlay and takes
no width out of the wrap.
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