A picked raster is now classified from its HEADER BYTES ONLY before anything
heavy happens: src/backdrop-probe.ts parses PNG IHDR (+colour type/tRNS for
alpha), JPEG SOF and WebP VP8/VP8L/VP8X at fixed offsets, never using a file
field as an allocation size; hostile or truncated headers collapse to
'unknown', which warns without numbers instead of passing silently. The
thresholds live in that module as the single calibration point
(WARN_DECODED_BYTES 128 MiB ≈ 32 MP, HARD_DIMENSION 16384 — the browser
canvas cap, DOWNSCALE_TARGET_PX 4096), derived from the desktop-Chromium
matrix now committed as demo/benchmark_backdrop_decode.mjs with a
conservative tablet margin documented in the spec.
The shared pick flow (src/backdrop-pick.ts) feeds BOTH lazy runtimes — the
editor space dialog and the onboarding first-space dialog — so the guard
cannot drift between them, and nothing of it enters the eager View graph.
Warn shows the real numbers and three actions; the reduced copy decodes
EXIF-aware, keeps aspect and alpha (PNG stays PNG, opaque becomes JPEG
q0.9) and flows through the ordinary planFile → upload path. Hard has two
phases with one outcome: beyond 16384 px only Cancel; a failed or timed-out
(10 s) reduce closes with a toast, clean staging and NO silent fallback to
the original the user just declined. SVG never reaches the probe. The safe
path swaps the manual byte-loop base64 for FileReader — half the JS-heap
peak on every upload, byte-identical output (parity asserted in the smoke).
Proofs: header-table units incl. a fuzz set of hostile headers and ±1
threshold bounds; smoke_backdrop_guard on the real bundle — zero decode
calls before the choice, byte parity of keep-original, a real 6200 px
reduce to 4096 for both alpha and opaque branches, cancel-only hard
dialog, both phase-2 failures (reject and hang under the test-only timeout
override), re-pick after refusal, SVG bypass; four registry mutants
(probe-always-safe, alpha-dropped, hard-demoted, phase-2 silent fallback).
Spec anchor corrected alongside: the server plan limit is 8 MB
(MAX_PLAN_BYTES), attachments are the 50 MB path — an 8 MB JPEG is easily
80-160 MP decoded, so the client-side guard stays the primary defence.
Issue: #39
User-Visible: yes
Sergey Matyunin