perf: RDP dirty rectangle tracking — partial frame transfer

Root cause: Every GraphicsUpdate copied the full 8.3MB decoded image
into the front buffer, cloned all 8.3MB for IPC, and transferred all
8.3MB to the frontend — even when only a 100x100 pixel region changed.
During window drag, this created a 25MB/frame pipeline backup.

Fix:
- Track dirty rectangles from ironrdp's GraphicsUpdate(InclusiveRectangle)
- Write path: only copy changed rows from decoded image to front buffer
  (e.g. 100 rows × 1920 pixels = 768KB vs 8.3MB full frame)
- Accumulate dirty region as union of all rects since last get_frame
- Read path: if dirty region < 50% of frame, extract only the dirty
  rectangle bytes; otherwise fall back to full frame
- Binary IPC format: 8-byte header [x,y,w,h as u16 LE] + pixel data
- Frontend: putImageData at dirty rect offset instead of full frame
- Status bar: h-9 text-sm for 3440x1440 readability

During window drag (typical 300x400 dirty rect):
  Before: 8.3MB write + 8.3MB clone + 8.3MB IPC = 24.9MB per frame
  After:  480KB write + 480KB extract + 480KB IPC = 1.4MB per frame
  ~17x reduction in data movement per frame.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/rdp_commands.rs

@@ -19,18 +19,37 @@ pub fn connect_rdp(
     state.rdp.connect(config, app_handle)
 }
 
-/// Get the current frame buffer as raw RGBA bytes via binary IPC.
+/// Get the dirty region since last call as raw RGBA bytes via binary IPC.
 ///
-/// Uses `tauri::ipc::Response` to return raw bytes without JSON serialization.
-/// Pixel format: RGBA, 4 bytes per pixel, row-major, top-left origin.
-/// Returns empty payload if frame hasn't changed since last call.
+/// Binary format: 8-byte header + pixel data
+///   Header: [x: u16, y: u16, width: u16, height: u16] (little-endian)
+///   If header is all zeros, the payload is a full frame (width*height*4 bytes).
+///   If header is non-zero, payload contains only the dirty rectangle pixels.
+/// Returns empty payload if nothing changed.
 #[tauri::command]
 pub fn rdp_get_frame(
     session_id: String,
     state: State<'_, AppState>,
 ) -> Result<Response, String> {
-    let frame = state.rdp.get_frame(&session_id)?;
-    Ok(Response::new(frame))
+    let (region, pixels) = state.rdp.get_frame(&session_id)?;
+    if pixels.is_empty() {
+        return Ok(Response::new(Vec::new()));
+    }
+    // Prepend 8-byte dirty rect header
+    let mut out = Vec::with_capacity(8 + pixels.len());
+    match region {
+        Some(rect) => {
+            out.extend_from_slice(&rect.x.to_le_bytes());
+            out.extend_from_slice(&rect.y.to_le_bytes());
+            out.extend_from_slice(&rect.width.to_le_bytes());
+            out.extend_from_slice(&rect.height.to_le_bytes());
+        }
+        None => {
+            out.extend_from_slice(&[0u8; 8]); // full frame marker
+        }
+    }
+    out.extend_from_slice(&pixels);
+    Ok(Response::new(out))
 }
 
 /// Send a mouse event to an RDP session.

src-tauri/src/rdp/mod.rs

@@ -66,14 +66,25 @@ enum InputEvent {
     Disconnect,
 }
 
+/// Dirty rectangle from the last GraphicsUpdate — used for partial frame transfer.
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct DirtyRect {
+    pub x: u16,
+    pub y: u16,
+    pub width: u16,
+    pub height: u16,
+}
+
 struct RdpSessionHandle {
     id: String,
     hostname: String,
     width: u16,
     height: u16,
     /// Frame buffer: RDP thread writes via RwLock write, IPC reads via RwLock read.
-    /// Brief write-lock per GraphicsUpdate, concurrent reads for get_frame.
     front_buffer: Arc<std::sync::RwLock<Vec<u8>>>,
+    /// Accumulated dirty region since last get_frame — union of all GraphicsUpdate rects.
+    dirty_region: Arc<std::sync::Mutex<Option<DirtyRect>>>,
     frame_dirty: Arc<AtomicBool>,
     input_tx: mpsc::UnboundedSender<InputEvent>,
 }
@@ -102,6 +113,7 @@ impl RdpService {
             pixel[3] = 255;
         }
         let front_buffer = Arc::new(std::sync::RwLock::new(initial_buf));
+        let dirty_region = Arc::new(std::sync::Mutex::new(None));
         let frame_dirty = Arc::new(AtomicBool::new(false));
 
         let (input_tx, input_rx) = mpsc::unbounded_channel();
@@ -112,6 +124,7 @@ impl RdpService {
             width,
             height,
             front_buffer: front_buffer.clone(),
+            dirty_region: dirty_region.clone(),
             frame_dirty: frame_dirty.clone(),
             input_tx,
         });
@@ -159,6 +172,7 @@ impl RdpService {
                     connection_result,
                     framed,
                     front_buffer,
+                    dirty_region,
                     frame_dirty,
                     input_rx,
                     width as u16,
@@ -202,13 +216,43 @@ impl RdpService {
         Ok(session_id)
     }
 
-    pub fn get_frame(&self, session_id: &str) -> Result<Vec<u8>, String> {
+    /// Get the dirty region since the last call. Returns (region_metadata, pixel_bytes).
+    /// The pixel bytes contain only the dirty rectangle in row-major RGBA order.
+    /// If nothing changed, returns empty bytes. If the dirty region covers >50% of the
+    /// frame, falls back to full frame for efficiency (avoids row-by-row extraction).
+    pub fn get_frame(&self, session_id: &str) -> Result<(Option<DirtyRect>, Vec<u8>), String> {
         let handle = self.sessions.get(session_id).ok_or_else(|| format!("RDP session {} not found", session_id))?;
         if !handle.frame_dirty.swap(false, Ordering::Acquire) {
-            return Ok(Vec::new()); // No change — return empty
+            return Ok((None, Vec::new()));
         }
+
+        let region = handle.dirty_region.lock().unwrap_or_else(|e| e.into_inner()).take();
         let buf = handle.front_buffer.read().unwrap_or_else(|e| e.into_inner());
-        Ok(buf.clone())
+        let stride = handle.width as usize * 4;
+        let total_pixels = handle.width as usize * handle.height as usize;
+
+        match region {
+            Some(rect) if (rect.width as usize * rect.height as usize) < total_pixels / 2 => {
+                // Partial: extract only the dirty rectangle
+                let rw = rect.width as usize;
+                let rh = rect.height as usize;
+                let rx = rect.x as usize;
+                let ry = rect.y as usize;
+                let mut out = Vec::with_capacity(rw * rh * 4);
+                for row in ry..ry + rh {
+                    let start = row * stride + rx * 4;
+                    let end = start + rw * 4;
+                    if end <= buf.len() {
+                        out.extend_from_slice(&buf[start..end]);
+                    }
+                }
+                Ok((Some(rect), out))
+            }
+            _ => {
+                // Full frame: dirty region covers most of the screen or is missing
+                Ok((None, buf.clone()))
+            }
+        }
     }
 
     pub fn get_frame_raw(&self, session_id: &str) -> Result<Vec<u8>, String> {
@@ -342,7 +386,7 @@ async fn establish_connection(config: connector::Config, hostname: &str, port: u
     Ok((connection_result, upgraded_framed))
 }
 
-async fn run_active_session(connection_result: ConnectionResult, framed: UpgradedFramed, front_buffer: Arc<std::sync::RwLock<Vec<u8>>>, frame_dirty: Arc<AtomicBool>, mut input_rx: mpsc::UnboundedReceiver<InputEvent>, width: u16, height: u16, app_handle: tauri::AppHandle, session_id: String) -> Result<(), String> {
+async fn run_active_session(connection_result: ConnectionResult, framed: UpgradedFramed, front_buffer: Arc<std::sync::RwLock<Vec<u8>>>, dirty_region: Arc<std::sync::Mutex<Option<DirtyRect>>>, frame_dirty: Arc<AtomicBool>, mut input_rx: mpsc::UnboundedReceiver<InputEvent>, width: u16, height: u16, app_handle: tauri::AppHandle, session_id: String) -> Result<(), String> {
     let (mut reader, mut writer) = split_tokio_framed(framed);
     let mut image = DecodedImage::new(PixelFormat::RgbA32, width, height);
     let mut active_stage = ActiveStage::new(connection_result);
@@ -400,16 +444,43 @@ async fn run_active_session(connection_result: ConnectionResult, framed: Upgrade
         for out in outputs {
             match out {
                 ActiveStageOutput::ResponseFrame(frame) => { writer.write_all(&frame).await.map_err(|e| format!("Failed to write RDP response frame: {}", e))?; }
-                ActiveStageOutput::GraphicsUpdate(_region) => {
-                    // Write decoded image directly to front buffer.
-                    // Single RwLock write — readers use read lock, no contention.
+                ActiveStageOutput::GraphicsUpdate(region) => {
+                    let rx = region.left as usize;
+                    let ry = region.top as usize;
+                    let rr = (region.right as usize).saturating_add(1).min(width as usize);
+                    let rb = (region.bottom as usize).saturating_add(1).min(height as usize);
+                    let stride = width as usize * 4;
+
+                    // Copy only the dirty rectangle rows from decoded image → front buffer
                     {
                         let src = image.data();
                         let mut front = front_buffer.write().unwrap_or_else(|e| e.into_inner());
-                        if src.len() == front.len() { front.copy_from_slice(src); } else { *front = src.to_vec(); }
+                        for row in ry..rb {
+                            let src_start = row * stride + rx * 4;
+                            let src_end = row * stride + rr * 4;
+                            if src_end <= src.len() && src_end <= front.len() {
+                                front[src_start..src_end].copy_from_slice(&src[src_start..src_end]);
                             }
+                        }
+                    }
+
+                    // Accumulate dirty region (union of all rects since last get_frame)
+                    {
+                        let new_rect = DirtyRect { x: rx as u16, y: ry as u16, width: (rr - rx) as u16, height: (rb - ry) as u16 };
+                        let mut dr = dirty_region.lock().unwrap_or_else(|e| e.into_inner());
+                        *dr = Some(match dr.take() {
+                            None => new_rect,
+                            Some(prev) => {
+                                let x = prev.x.min(new_rect.x);
+                                let y = prev.y.min(new_rect.y);
+                                let r = (prev.x + prev.width).max(new_rect.x + new_rect.width);
+                                let b = (prev.y + prev.height).max(new_rect.y + new_rect.height);
+                                DirtyRect { x, y, width: r - x, height: b - y }
+                            }
+                        });
+                    }
+
                     frame_dirty.store(true, Ordering::Release);
-                    // Signal frontend — rAF coalescing prevents flood
                     let _ = app_handle.emit(&format!("rdp:frame:{}", session_id), ());
                 }
                 ActiveStageOutput::Terminate(reason) => { info!("RDP session terminated: {:?}", reason); return Ok(()); }

src/components/common/StatusBar.vue

@@ -1,5 +1,5 @@
 <template>
-  <div class="h-8 flex items-center justify-between px-4 bg-[var(--wraith-bg-secondary)] border-t border-[var(--wraith-border)] text-xs text-[var(--wraith-text-muted)] shrink-0">
+  <div class="h-9 flex items-center justify-between px-4 bg-[var(--wraith-bg-secondary)] border-t border-[var(--wraith-border)] text-sm text-[var(--wraith-text-muted)] shrink-0">
     <!-- Left: connection info -->
     <div class="flex items-center gap-3">
       <template v-if="sessionStore.activeSession">

src/composables/useRdp.ts

@@ -158,8 +158,8 @@ export interface UseRdpReturn {
   keyboardGrabbed: Ref<boolean>;
   /** Whether clipboard sync is enabled */
   clipboardSync: Ref<boolean>;
-  /** Fetch the current frame as RGBA ImageData */
-  fetchFrame: (sessionId: string, width: number, height: number) => Promise<ImageData | null>;
+  /** Fetch and render the dirty region directly to a canvas context */
+  fetchAndRender: (sessionId: string, width: number, height: number, ctx: CanvasRenderingContext2D) => Promise<boolean>;
   /** Send a mouse event to the backend */
   sendMouse: (sessionId: string, x: number, y: number, flags: number) => void;
   /** Send a key event to the backend */
@@ -199,38 +199,50 @@ export function useRdp(): UseRdpReturn {
   let unlistenFrame: (() => void) | null = null;
 
   /**
-   * Fetch the current frame from the Rust RDP backend.
+   * Fetch the dirty region from the Rust RDP backend and apply it to the canvas.
    *
-   * rdp_get_frame returns raw RGBA bytes (width*height*4) serialised as a
-   * base64 string over Tauri's IPC bridge. We decode it to Uint8ClampedArray
-   * and wrap in an ImageData for putImageData().
+   * Binary format from backend: 8-byte header + pixel data
+   *   Header: [x: u16, y: u16, w: u16, h: u16] (little-endian)
+   *   If header is all zeros → full frame (width*height*4 bytes)
+   *   If header is non-zero → dirty rectangle (w*h*4 bytes)
+   *
+   * Returns true if a frame was rendered, false if nothing changed.
    */
-  async function fetchFrame(
+  async function fetchAndRender(
     sessionId: string,
     width: number,
     height: number,
-  ): Promise<ImageData | null> {
+    ctx: CanvasRenderingContext2D,
+  ): Promise<boolean> {
     let raw: ArrayBuffer;
     try {
       raw = await invoke<ArrayBuffer>("rdp_get_frame", { sessionId });
     } catch {
-      return null;
+      return false;
     }
 
-    if (!raw || raw.byteLength === 0) return null;
+    if (!raw || raw.byteLength <= 8) return false;
 
-    // Binary IPC — tauri::ipc::Response delivers raw bytes as ArrayBuffer
-    const bytes = new Uint8ClampedArray(raw);
+    const view = new DataView(raw);
+    const rx = view.getUint16(0, true);
+    const ry = view.getUint16(2, true);
+    const rw = view.getUint16(4, true);
+    const rh = view.getUint16(6, true);
+    const pixelData = new Uint8ClampedArray(raw, 8);
 
+    if (rx === 0 && ry === 0 && rw === 0 && rh === 0) {
+      // Full frame
       const expected = width * height * 4;
-    if (bytes.length !== expected) {
-      console.warn(
-        `[useRdp] Frame size mismatch: got ${bytes.length}, expected ${expected}`,
-      );
-      return null;
+      if (pixelData.length !== expected) return false;
+      ctx.putImageData(new ImageData(pixelData, width, height), 0, 0);
+    } else {
+      // Dirty rectangle — apply at offset
+      const expected = rw * rh * 4;
+      if (pixelData.length !== expected) return false;
+      ctx.putImageData(new ImageData(pixelData, rw, rh), rx, ry);
     }
 
-    return new ImageData(bytes, width, height);
+    return true;
   }
 
   /**
@@ -301,7 +313,7 @@ export function useRdp(): UseRdpReturn {
     let fetchPending = false;
     let rafScheduled = false;
 
-    // Fetch frame when backend signals a new frame is ready.
+    // Fetch and render dirty region when backend signals new frame data.
     // Uses rAF to coalesce rapid events into one fetch per display frame.
     function scheduleFrameFetch(): void {
       if (rafScheduled) return;
@@ -310,12 +322,9 @@ export function useRdp(): UseRdpReturn {
         rafScheduled = false;
         if (fetchPending) return;
         fetchPending = true;
-        const imageData = await fetchFrame(sessionId, width, height);
+        const rendered = await fetchAndRender(sessionId, width, height, ctx);
         fetchPending = false;
-        if (imageData && ctx) {
-          ctx.putImageData(imageData, 0, 0);
-          if (!connected.value) connected.value = true;
-        }
+        if (rendered && !connected.value) connected.value = true;
       });
     }
 
@@ -363,7 +372,7 @@ export function useRdp(): UseRdpReturn {
     connected,
     keyboardGrabbed,
     clipboardSync,
-    fetchFrame,
+    fetchAndRender,
     sendMouse,
     sendKey,
     sendClipboard,