The Mac Swift app (hermes-webui/hermes-swift-mac) and any other native
WKWebView wrapper need the active theme background to keep AppKit
chrome (tab bar, title bar, traffic-light area) in sync with the page.
The current Mac approach pixel-samples the page via
elementsFromPoint, which is fragile against modals/lightboxes/file-tree
overlays — any opaque overlay over a sample point can poison the
chrome colour for the entire app. (See swift-mac issue #70.)
Surface the active theme's background as the canonical, overlay-resistant
source of truth via <meta name="theme-color">:
- Two static prefers-color-scheme variants in <head> for browsers that
read theme-color before any JS runs (mobile Safari, PWAs).
- One id="hermes-theme-color" runtime tag with an inline pre-paint
seed script that reads localStorage hermes-theme so the meta tag
is correct on first paint, before boot.js loads.
- New _syncThemeColorMeta() helper in static/boot.js that reads
getComputedStyle(html).getPropertyValue('--bg') and writes it into
the runtime meta tag. Called from _setResolvedTheme (both branches —
prism-loaded and prism-absent) and from _applySkin so every theme
toggle and skin switch updates the meta tag.
Reading --bg via getComputedStyle means each skin (Default, Sienna,
Sisyphus, Charizard, etc.) reaches the meta tag with its distinct
background — no per-skin lookup table to drift.
Browser-verified end to end on port 8789:
- light + default → meta=#FEFCF7 (matches --bg)
- light + Sienna → meta=#FAF9F5 (skin's distinct bg)
- dark + Sienna → meta=#1F1E1C (skin's dark variant)
10 regression tests added in tests/test_theme_color_meta_bridge.py
covering: static media variants present, runtime id stable, pre-paint
seed reads localStorage, helper defined and reads computed --bg,
helper targets known id, both _setResolvedTheme branches call sync,
_applySkin calls sync, root --bg defaults still match.
Companion PR coming on hermes-webui/hermes-swift-mac to switch the
theme bridge from elementsFromPoint pixel-sampling to reading
document.querySelector('meta[name="theme-color"][id="hermes-theme-color"]').content.
Refs hermes-webui/hermes-swift-mac#70.
e9aac079e1
