fix(call): Response return-type can't be wrapped in Result

Previous commit had the command typed as Result<tauri::ipc::Response,
String>. Turns out that forces Tauri to JSON-serialise the variant
wrapper around the Response body — the frontend gets a JSON object
instead of the raw ArrayBuffer, the runtime check for byteLength fails,
and every thumbnail comes back as null.

Changed the return type to `tauri::ipc::Response` directly. Bad source
ids and capture failures now funnel into an empty byte buffer; the JS
side still detects "no thumbnail" via `byteLength === 0` so the
contract stays the same.

Frontend also widens the invoke-result typing to ArrayBuffer |
Uint8Array | number[] so an older WebView2 that happens to deserialise
as an array still works, and normalises into a plain ArrayBuffer
before constructing the Blob to sidestep a TS SharedArrayBuffer
incompatibility.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/desktop/src-tauri/src/screen_sources.rs

@@ -83,22 +83,23 @@ pub fn capture_screen_source_thumbnail(source_id: String) -> Result<Option<Strin
 // expose via `URL.createObjectURL` — skips the base64-decode step
 // entirely and keeps the main thread responsive during fan-in.
 //
-// A capture failure (source vanished, permission denied) returns an empty
-// byte buffer rather than an error so the JS side gets a uniform contract
-// (ArrayBuffer always). Caller checks `byteLength === 0` to detect the
-// no-thumbnail case.
+// The return type MUST be `Response` directly (not `Result<Response, E>`):
+// a Result wrapper forces Tauri to JSON-serialise the variant so the
+// frontend gets a JSON object instead of raw bytes. Failures — bad id
+// format, capture errors, source vanished — all funnel into an empty
+// byte buffer; the caller treats `byteLength === 0` as the "no thumbnail"
+// signal.
 #[tauri::command]
-pub fn capture_screen_source_thumbnail_bytes(
-    source_id: String,
-) -> Result<tauri::ipc::Response, String> {
+pub fn capture_screen_source_thumbnail_bytes(source_id: String) -> tauri::ipc::Response {
     let bytes = if let Some(raw) = source_id.strip_prefix("screen:").and_then(strip_zero_suffix) {
         capture_monitor_bytes_by_id(raw).unwrap_or_default()
     } else if let Some(raw) = source_id.strip_prefix("window:").and_then(strip_zero_suffix) {
         capture_window_bytes_by_id(raw).unwrap_or_default()
     } else {
-        return Err(format!("unknown source id format: {source_id}"));
+        eprintln!("capture_screen_source_thumbnail_bytes: unknown id format: {source_id}");
+        Vec::new()
     };
-    Ok(tauri::ipc::Response::new(bytes))
+    tauri::ipc::Response::new(bytes)
 }
 
 fn strip_zero_suffix(s: &str) -> Option<&str> {

apps/desktop/src/lib/screenSources.ts

@@ -60,18 +60,41 @@ export async function captureScreenSourceThumbnail(
 // in a Blob so callers can hand it straight to `URL.createObjectURL` —
 // never touches base64 on either side. Returns null when the Rust side
 // produced zero bytes (capture failed, source vanished).
+//
+// Tauri's invoke may resolve with different shapes depending on runtime
+// version — accept ArrayBuffer, Uint8Array, or a number[] fallback so a
+// WebView2 variant that doesn't speak the binary channel still works.
 export async function captureScreenSourceThumbnailBytes(
   sourceId: string,
 ): Promise<Blob | null> {
   if (!isTauriRuntime()) return null;
   try {
     const { invoke } = await import('@tauri-apps/api/core');
-    const result = await invoke<ArrayBuffer>(
+    const result = await invoke<ArrayBuffer | Uint8Array | number[]>(
       'capture_screen_source_thumbnail_bytes',
       { sourceId },
     );
-    if (!result || result.byteLength === 0) return null;
-    return new Blob([result], { type: 'image/jpeg' });
+    let bytes: ArrayBuffer | Uint8Array | null = null;
+    if (result instanceof ArrayBuffer) {
+      bytes = result;
+    } else if (result instanceof Uint8Array) {
+      bytes = result;
+    } else if (Array.isArray(result) && result.length > 0) {
+      bytes = new Uint8Array(result);
+    }
+    if (!bytes || bytes.byteLength === 0) {
+      return null;
+    }
+    // Normalise to ArrayBuffer so Blob() accepts the part without TS
+    // complaining about SharedArrayBuffer-backed Uint8Array flavours.
+    const buffer =
+      bytes instanceof ArrayBuffer
+        ? bytes
+        : (bytes.buffer.slice(
+            bytes.byteOffset,
+            bytes.byteOffset + bytes.byteLength,
+          ) as ArrayBuffer);
+    return new Blob([buffer], { type: 'image/jpeg' });
   } catch (err: unknown) {
     console.warn('capture_screen_source_thumbnail_bytes failed', { sourceId, err });
     return null;