Mask unstable content in snapshots with snapshotMasked()

[BarredEwe]

Dynamic content — dates, timers, remote avatars, random data — makes snapshot tests flaky, and teams end up disabling them. This adds a way to replace such areas with a fixed plate before comparison.

Text(Date.now.formatted()).snapshotMasked()
AvatarView(url: url).snapshotMasked(color: .prefireSnapshotMask)

Applying the modifier to a container masks its whole subtree with a single plate, which also covers the "mask the entire preview" case.

Implementation

SnapshotMaskModifier reads an internal environment value isPrefireSnapshotRendering. While rendering a snapshot it draws content.hidden().overlay(color) — .hidden() keeps the view in the layout pass, so its size and the surrounding layout are unchanged and the overlay fills exactly that frame. No image post-processing, no redacted heuristics.

The environment value is set in PrefireSnapshot.loadViewWithPreferences() — the single choke point both platform paths go through, so iOS/tvOS and macOS are covered by one line. PlaybookView renders PreviewModel.content directly and never touches PrefireSnapshot, so the Playbook keeps real content. Outside a snapshot the modifier returns the content untouched, so it stays a no-op in the Xcode canvas.

No generator or config changes: masking is purely a runtime concern.

Tests

Five pixel-level tests in MacOSSnapshotTests: masked views with different text produce byte-identical PNGs; the same views unmasked differ (the control proving rendering captures content); the masked area's center pixel matches the plate color; masked and unmasked hosts have identical layout size; and the same masked view hosted outside PrefireSnapshot still renders its real content.

make test 56/56, make test-cli 22/22, make build clean.