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skills(headless-screenshot): document two false-positive traps

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Hit both today doing a live QA pass on the gateway: plain
nixpkgs#chromium has no color-emoji font (tofu boxes read exactly
like a missing-icon bug), and a page whose content loads async after
the initial paint (SSE, fetch-on-mount) screenshots as a bare
"loading..." placeholder without --virtual-time-budget. Neither is a
real product bug, but both need a documented fix for when an accurate
render actually matters, not just "know to discount it."
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iris 2026-08-15 09:38:08 +02:00 • committed by mara
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33
claude-plugins/plugins/base/skills/headless-screenshot/SKILL.md
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@ -69,3 +69,36 @@ question is genuinely about *behavior*, not appearance. A missing
emoji/glyph rendering as a box in the screenshot is usually a font
availability artifact of the headless environment, not a real bug -
don't mistake one for the other when reporting results.
## Two false-positive traps (and their fixes)
Both of these look exactly like a real product bug in a naive
screenshot and aren't - check for them before filing anything, and
use the fix below if you actually need an accurate render (e.g. to
judge icon/glyph layout, not just to note "there'd be an icon here").
- **Tofu boxes instead of emoji.** Plain `nixpkgs#chromium` has no
color-emoji font, so any 🖥/📡/🔨-style glyph renders as an empty
box. Add `nixpkgs#noto-fonts-color-emoji` to the `nix shell` - but a
`FONTCONFIG_FILE` that lists *only* the emoji dir throws away the
base fonts and blanks all normal text instead, which is its own
false alarm. Merge, don't override:
```
cat > fonts.conf <<EOF
<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>
<include ignore_missing="yes">/etc/fonts/fonts.conf</include>
<dir>$(nix eval --raw nixpkgs#noto-fonts-color-emoji)/share/fonts</dir>
</fontconfig>
EOF
FONTCONFIG_FILE=$PWD/fonts.conf nix shell nixpkgs#chromium nixpkgs#noto-fonts-color-emoji \
--command chromium --headless=new --disable-gpu --no-sandbox \
--screenshot=out.png --window-size=1440,900 "https://example/"
```
- **A bare "loading..." placeholder instead of real content.** A page
whose content arrives after the initial paint (SSE, a `fetch` on
mount) can get captured before that data lands - the default
`--screenshot` doesn't wait for it. Add
`--virtual-time-budget=8000` (milliseconds; tune to the page) to let
it actually settle first.