fix: preserve client-side brightness on monitor refetch (v6.4.5)

Sliding a monitor's brightness occasionally snapped back because
get_monitors / fetch_all_state reads brightness from the monitor over
DDC, and DDC takes time to settle. A refetch triggered during that
window (popup re-show, visibilitychange, monitors-changed event) would
return a stale value and clobber the user's intended setting.

Make the client the source of truth for brightness/contrast: when
refetching, merge by uid and keep the existing brightness/contrast
values. Monitor list, names, hidden flag, and contrast capability still
come from the backend.

Co-Authored-By: Claude Opus 4 (1M context) <noreply@anthropic.com>

Author: synle

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://raw.githubusercontent.com/nicoverbruggen/tauri-v2-docs/refs/heads/main/schemas/config.schema.json",
   "productName": "Display DJ",
-  "version": "6.4.4",
+  "version": "6.4.5",
   "identifier": "com.synle.display-dj",
   "build": {
     "beforeDevCommand": "npm run dev",

src/App.tsx

@@ -31,15 +31,33 @@ function App() {
   const [version, setVersion] = useState('');
   const appRef = useRef<HTMLDivElement>(null);
 
+  /** Merges refetched monitors with the current state, preserving client-side
+   * brightness/contrast values. The client is the source of truth for these —
+   * reading them back from the sidecar can return stale values while DDC
+   * settles, which causes the slider to snap back to the pre-change value. */
+  const mergeMonitors = useCallback((fetched: Monitor[], prev: Monitor[]): Monitor[] => {
+    return fetched.map((m) => {
+      const existing = prev.find((p) => p.uid === m.uid);
+      if (!existing) return m;
+      return {
+        ...m,
+        brightness: existing.brightness,
+        // Only preserve contrast when both sides still report DDC capability
+        contrast:
+          m.contrast !== null && existing.contrast !== null ? existing.contrast : m.contrast,
+      };
+    });
+  }, []);
+
   /** Fetches the list of connected monitors from the backend. */
   const fetchMonitors = useCallback(async () => {
     try {
       const m = await invoke<Monitor[]>('get_monitors');
-      setMonitors(m);
+      setMonitors((prev) => mergeMonitors(m, prev));
     } catch (e) {
       console.error('Failed to get monitors:', e);
     }
-  }, []);
+  }, [mergeMonitors]);
 
   /** Fetches the current dark mode state from the backend. */
   const fetchDarkMode = useCallback(async () => {
@@ -91,13 +109,15 @@ function App() {
         isDark: boolean;
         volume: number;
       }>('fetch_all_state');
-      setMonitors(state.monitors);
+      setMonitors((prev) =>
+        prev.length === 0 ? state.monitors : mergeMonitors(state.monitors, prev),
+      );
       setDarkMode(state.isDark);
       setVolume(state.volume);
     } catch (e) {
       console.error('Failed to fetch all state:', e);
     }
-  }, []);
+  }, [mergeMonitors]);
 
   useEffect(() => {
     fetchAllState();