Font spacing perturbation has no off switch (unlike audio/canvas)

[rubenvereecken]

Related: #76 — PixelScan reports 5 checks; this is the fix for the "Fingerprint: Masking detected" one specifically. The other four (Browser, Location, Proxy, Bot check) are unrelated to font spacing — see below.

Problem

AudioFingerprintManager::ApplyTransformation() has a clean off switch — when seed == 0, it returns early and produces vanilla audio output. Font spacing perturbation doesn't have this: when no seed is set, gfxHarfBuzzShaper.cpp falls back to a hardcoded constant (0x6D2B79F5u) and always applies perturbation.

This means every Camoufox session produces font metrics and canvas text hashes that no real device has ever generated. Fingerprint checkers that maintain a population database (PixelScan's "Masking detected" verdict) can trivially flag this as synthetic.

Perturbation	seed == 0 behaviour	Hardcoded fallback	Can disable from Python?
Audio	returns early, no-op	none	yes — set audio:seed to 0
Font spacing	LCG still runs (0 → 12345)	0x6D2B79F5u when no seed set	no
Canvas	returns early, no-op	none	yes — set canvas:seed to 0

The audio and canvas paths let you opt out. Font spacing doesn't — inconsistent and makes A/B testing impossible from the Python API.

Evidence

PixelScan runs 5 checks. With the font spacing fallback removed, the Fingerprint check flips from fail to pass:

PixelScan check	Before fix	After fix	Notes
Browser	pass	pass	Firefox correctly identified
Location	pass	pass	Geoip/timezone cross-check
Proxy	fail (when proxied)	fail (when proxied)	IP reputation — unrelated
Fingerprint	fail — "Masking detected"	pass	Font spacing was the trigger
Bot check	pass	pass	Juggler protocol not flagged

Tested across 6 Camoufox runs + 1 vanilla Firefox baseline. A custom build with the hardcoded fallback removed passes the Fingerprint check cleanly.

Current C++ behaviour

// gfxHarfBuzzShaper.cpp — ShapeText()
uint32_t seed = FontSpacingSeedManager::GetSeed(pbid);
bool seedFromManager = (seed != 0);
if (!seedFromManager) {
    seed = 0x6D2B79F5u; // always-on fallback
}
// LCG + spacing applied unconditionally

Compare with audio:

// AudioFingerprintManager::ApplyTransformation()
if (seed == 0 || length == 0 || !data) {
    return;  // clean no-op
}

Suggested fix

Remove the hardcoded fallback and guard the perturbation block, same as audio:

uint32_t seed = FontSpacingSeedManager::GetSeed(pbid);

if (seed != 0) {
    // LCG + apply spacing (existing code, just wrapped in the guard)
}

When no seed is set, GetSeed() returns 0, the guard skips perturbation, and you get vanilla font metrics. To disable from the Python API, set fonts:spacing_seed to 0 in the config/preset — same convention as audio:seed and canvas:seed.

Happy to put up a PR if this looks reasonable.