Follow the OS appearance when set to System without a new window (#540)

packages/theme/src/appearance.ts

@@ -4,6 +4,21 @@ export type Appearance = "light" | "dark";
 
 const SYSTEM_APPEARANCE_CHANGE_EVENT = "system_appearance_change";
 
+declare global {
+  interface Window {
+    __YAAK_INITIAL_APPEARANCE__?: Appearance;
+  }
+}
+
+// NOTE: On macOS and Linux the backend detects the appearance the OS prefers and injects
+//  it here, later emitting change events. This is required because applying a theme forces
+//  the window appearance, which poisons what the webview reports (CSS media queries and
+//  window theme events follow the override).
+export function getSystemAppearance(): Appearance | null {
+  const appearance = window.__YAAK_INITIAL_APPEARANCE__;
+  return appearance === "dark" || appearance === "light" ? appearance : null;
+}
+
 export function getCSSAppearance(): Appearance {
   return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
 }
@@ -41,12 +56,24 @@ export function resolveAppearance(
 }
 
 export function subscribeToPreferredAppearance(cb: (appearance: Appearance) => void) {
+  const systemAppearance = getSystemAppearance();
+  if (systemAppearance != null) {
+    cb(systemAppearance);
+    return subscribeToSystemAppearanceChange(cb);
+  }
+
   cb(getCSSAppearance());
   void getWindowAppearance().then(cb);
   return subscribeToPreferredAppearanceChange(cb);
 }
 
 export function subscribeToPreferredAppearanceChange(cb: (appearance: Appearance) => void) {
+  if (getSystemAppearance() != null) {
+    // The CSS and window sources are poisoned by forced window appearances, so only
+    // listen to the backend's system appearance detection when it's available
+    return subscribeToSystemAppearanceChange(cb);
+  }
+
   const unsubscribeCSS = subscribeToCSSAppearanceChange(cb);
   const unsubscribeWindow = subscribeToWindowAppearanceChange(cb);
   const unsubscribeSystem = subscribeToSystemAppearanceChange(cb);