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Same alpha-string trap pattern as the recent detectPageTheme fix, on a
different code path. resolveCanvasBackground walks parents looking for
an opaque background; on a page that doesn't set its own bg the loop
runs out and fell through to:
return getComputedStyle(document.body).backgroundColor
|| getComputedStyle(document.documentElement).backgroundColor
|| '#ffffff';
`getComputedStyle(body).backgroundColor` for a default-bg page returns
the literal string "rgba(0, 0, 0, 0)" — non-empty, truthy — so the `||`
chain short-circuits to transparent-black instead of falling through to
'#ffffff'. modern-screenshot then composites the capture onto a black
canvas; the WebGL shader overlay flashes solid black until the shader
finishes loading.
Fix: drop the buggy fallback. The while-loop already covered <body> and
<html>; if neither is opaque the only sensible answer is the browser's
default canvas color (white).
Test coverage:
- New tests/live-browser-regression.test.mjs pins the anti-pattern
with a static-source check (live-browser.js is an IIFE with no module
exports, so this is the cheapest reliable regression guard). Also
pins the equivalent guard for detectPageTheme's readOpaque helper
added in the prior commit.
- Wired the new test file into `bun run test`'s explicit list.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>