Fix video export resolution scaling and post-export UI hang

- Scale the document to the selected output resolution. It was rendered
  at document size regardless of the export dimensions, so picking a
  different resolution didn't scale the stage.
- Run the audio+video mux on a background thread instead of the UI
  thread, keeping the app responsive (showing "Finalizing") during the
  re-mux pass.
- Send desktop notifications fire-and-forget. notify_rust's show() is a
  synchronous D-Bus call that blocked the UI for the full service
  activation timeout (~25s) when no notification daemon is running.

lightningbeam-ui/lightningbeam-editor/src/export/mod.rs

@@ -113,10 +113,10 @@ struct ParallelExportState {
     video_progress_rx: Receiver<ExportProgress>,
     /// Audio progress channel
     audio_progress_rx: Receiver<ExportProgress>,
-    /// Video encoder thread handle
-    video_thread: std::thread::JoinHandle<()>,
-    /// Audio export thread handle
-    audio_thread: std::thread::JoinHandle<()>,
+    /// Video encoder thread handle (taken when the mux thread is spawned).
+    video_thread: Option<std::thread::JoinHandle<()>>,
+    /// Audio export thread handle (taken when the mux thread is spawned).
+    audio_thread: Option<std::thread::JoinHandle<()>>,
     /// Temporary video file path
     temp_video_path: PathBuf,
     /// Temporary audio file path
@@ -127,6 +127,9 @@ struct ParallelExportState {
     video_progress: Option<ExportProgress>,
     /// Latest audio progress
     audio_progress: Option<ExportProgress>,
+    /// Result channel for the background mux. `Some` once muxing has started; the
+    /// mux runs off the UI thread so the app stays responsive during finalization.
+    mux_rx: Option<Receiver<Result<(), String>>>,
 }
 
 impl ExportOrchestrator {
@@ -220,47 +223,33 @@ impl ExportOrchestrator {
             parallel.audio_progress = Some(progress);
         }
 
-        // Check if both are complete
-        let video_complete = matches!(parallel.video_progress, Some(ExportProgress::Complete { .. }));
-        let audio_complete = matches!(parallel.audio_progress, Some(ExportProgress::Complete { .. }));
-
-        if video_complete && audio_complete {
-            println!("🎬🎵 [PARALLEL] Both video and audio complete, starting mux");
-
-            // Take parallel state to extract file paths
-            let parallel_state = self.parallel_export.take().unwrap();
-
-            // Wait for threads to finish
-            parallel_state.video_thread.join().ok();
-            parallel_state.audio_thread.join().ok();
-
-            // Start muxing
-            match Self::mux_video_and_audio(
-                &parallel_state.temp_video_path,
-                &parallel_state.temp_audio_path,
-                &parallel_state.final_output_path,
-            ) {
-                Ok(()) => {
+        // If a background mux is already running, poll it without blocking the UI.
+        if parallel.mux_rx.is_some() {
+            match parallel.mux_rx.as_ref().unwrap().try_recv() {
+                Ok(Ok(())) => {
                     println!("✅ [MUX] Muxing complete, cleaning up temp files");
-
-                    // Clean up temp files
-                    std::fs::remove_file(&parallel_state.temp_video_path).ok();
-                    std::fs::remove_file(&parallel_state.temp_audio_path).ok();
-
-                    return Some(ExportProgress::Complete {
-                        output_path: parallel_state.final_output_path,
-                    });
+                    let state = self.parallel_export.take().unwrap();
+                    std::fs::remove_file(&state.temp_video_path).ok();
+                    std::fs::remove_file(&state.temp_audio_path).ok();
+                    return Some(ExportProgress::Complete { output_path: state.final_output_path });
                 }
-                Err(err) => {
+                Ok(Err(err)) => {
                     println!("❌ [MUX] Muxing failed: {}", err);
-                    return Some(ExportProgress::Error {
-                        message: format!("Muxing failed: {}", err),
-                    });
+                    self.parallel_export = None;
+                    return Some(ExportProgress::Error { message: format!("Muxing failed: {}", err) });
+                }
+                Err(std::sync::mpsc::TryRecvError::Empty) => {
+                    // Still muxing — keep the UI responsive and show finalizing state.
+                    return Some(ExportProgress::Finalizing);
+                }
+                Err(std::sync::mpsc::TryRecvError::Disconnected) => {
+                    self.parallel_export = None;
+                    return Some(ExportProgress::Error { message: "Mux thread terminated unexpectedly".to_string() });
                 }
             }
         }
 
-        // Check for errors
+        // Check for errors before completion.
         if let Some(ExportProgress::Error { ref message }) = parallel.video_progress {
             return Some(ExportProgress::Error { message: format!("Video: {}", message) });
         }
@@ -268,6 +257,30 @@ impl ExportOrchestrator {
             return Some(ExportProgress::Error { message: format!("Audio: {}", message) });
         }
 
+        // Both streams done → spawn the mux on a background thread (the previous
+        // implementation muxed synchronously here on the UI thread, which froze the
+        // app for the whole re-mux pass after progress already hit 100%).
+        let video_complete = matches!(parallel.video_progress, Some(ExportProgress::Complete { .. }));
+        let audio_complete = matches!(parallel.audio_progress, Some(ExportProgress::Complete { .. }));
+        if video_complete && audio_complete {
+            println!("🎬🎵 [PARALLEL] Both video and audio complete, starting background mux");
+            let video_thread = parallel.video_thread.take();
+            let audio_thread = parallel.audio_thread.take();
+            let video_path = parallel.temp_video_path.clone();
+            let audio_path = parallel.temp_audio_path.clone();
+            let output_path = parallel.final_output_path.clone();
+            let (tx, rx) = std::sync::mpsc::channel();
+            std::thread::spawn(move || {
+                // The export threads have signalled Complete; join is near-instant.
+                if let Some(t) = video_thread { t.join().ok(); }
+                if let Some(t) = audio_thread { t.join().ok(); }
+                let result = Self::mux_video_and_audio(&video_path, &audio_path, &output_path);
+                tx.send(result).ok();
+            });
+            parallel.mux_rx = Some(rx);
+            return Some(ExportProgress::Finalizing);
+        }
+
         // Return combined progress
         match (&parallel.video_progress, &parallel.audio_progress) {
             (Some(ExportProgress::FrameRendered { frame, total }), _) => {
@@ -979,13 +992,14 @@ impl ExportOrchestrator {
         self.parallel_export = Some(ParallelExportState {
             video_progress_rx,
             audio_progress_rx,
-            video_thread,
-            audio_thread,
+            video_thread: Some(video_thread),
+            audio_thread: Some(audio_thread),
             temp_video_path,
             temp_audio_path,
             final_output_path: output_path,
             video_progress: None,
             audio_progress: None,
+            mux_rx: None,
         });
 
         println!("🎬🎵 [PARALLEL EXPORT] Both threads spawned, ready for frames");

lightningbeam-ui/lightningbeam-editor/src/export/video_exporter.rs

@@ -982,8 +982,18 @@ pub fn render_frame_to_rgba_hdr(
     // Set document time to the frame timestamp
     document.current_time = timestamp;
 
-    // Use identity transform for export (document coordinates = pixel coordinates)
-    let base_transform = Affine::IDENTITY;
+    // Scale the document to the export resolution. The core renderer bakes this
+    // base transform into every layer (vector scenes, raster and video layer
+    // transforms), so the whole stage scales up/down to fill the output. When the
+    // export size matches the document this is the identity.
+    let base_transform = if document.width > 0.0 && document.height > 0.0 {
+        Affine::scale_non_uniform(
+            width as f64 / document.width,
+            height as f64 / document.height,
+        )
+    } else {
+        Affine::IDENTITY
+    };
 
     // Render document for compositing (returns per-layer scenes)
     let composite_result = render_document_for_compositing(
@@ -1189,8 +1199,18 @@ pub fn render_frame_to_gpu_rgba(
     // Set document time to the frame timestamp
     document.current_time = timestamp;
 
-    // Use identity transform for export (document coordinates = pixel coordinates)
-    let base_transform = Affine::IDENTITY;
+    // Scale the document to the export resolution. The core renderer bakes this
+    // base transform into every layer (vector scenes, raster and video layer
+    // transforms), so the whole stage scales up/down to fill the output. When the
+    // export size matches the document this is the identity.
+    let base_transform = if document.width > 0.0 && document.height > 0.0 {
+        Affine::scale_non_uniform(
+            width as f64 / document.width,
+            height as f64 / document.height,
+        )
+    } else {
+        Affine::IDENTITY
+    };
 
     // Render document for compositing (returns per-layer scenes)
     let composite_result = render_document_for_compositing(

lightningbeam-ui/lightningbeam-editor/src/main.rs

@@ -5616,11 +5616,8 @@ impl eframe::App for EditorApp {
                         // Close the progress dialog after a brief delay
                         self.export_progress_dialog.close();
 
-                        // Send desktop notification
-                        if let Err(e) = notifications::notify_export_complete(output_path) {
-                            // Log but don't fail - notifications are non-critical
-                            eprintln!("⚠️  Could not send desktop notification: {}", e);
-                        }
+                        // Send desktop notification (fire-and-forget; never blocks the UI)
+                        notifications::notify_export_complete(output_path);
                     }
                     lightningbeam_core::export::ExportProgress::Error { ref message } => {
                         eprintln!("❌ Export error: {}", message);
@@ -5630,11 +5627,8 @@ impl eframe::App for EditorApp {
                         );
                         // Keep the dialog open to show the error
 
-                        // Send desktop notification for error
-                        if let Err(e) = notifications::notify_export_error(message) {
-                            // Log but don't fail - notifications are non-critical
-                            eprintln!("⚠️  Could not send desktop notification: {}", e);
-                        }
+                        // Send desktop notification for error (fire-and-forget)
+                        notifications::notify_export_error(message);
                     }
                 }
             }

lightningbeam-ui/lightningbeam-editor/src/notifications.rs

@@ -3,52 +3,52 @@
 use notify_rust::Notification;
 use std::path::Path;
 
-/// Send a desktop notification for successful export
-///
-/// # Arguments
-/// * `output_path` - Path to the exported file
-///
-/// # Returns
-/// Ok(()) if notification sent successfully, Err with log message if failed
-pub fn notify_export_complete(output_path: &Path) -> Result<(), String> {
+// `notify_rust`'s `Notification::show()` is a SYNCHRONOUS D-Bus call. When no
+// notification daemon is running it can block for the service-activation timeout
+// (~25s) before failing, so these are fire-and-forget on a detached thread — the
+// UI must never wait on them (e.g. they're triggered from the export-complete
+// handler on the UI thread).
+
+/// Show a desktop notification for a successful export (fire-and-forget).
+pub fn notify_export_complete(output_path: &Path) {
     let filename = output_path
         .file_name()
         .and_then(|n| n.to_str())
-        .unwrap_or("unknown");
+        .unwrap_or("unknown")
+        .to_string();
 
-    Notification::new()
-        .summary("Export Complete")
-        .body(&format!("Successfully exported: {}", filename))
-        .icon("dialog-information")  // Standard icon name (freedesktop.org)
-        .timeout(5000)  // 5 seconds
-        .show()
-        .map_err(|e| format!("Failed to show notification: {}", e))?;
-
-    Ok(())
+    std::thread::spawn(move || {
+        if let Err(e) = Notification::new()
+            .summary("Export Complete")
+            .body(&format!("Successfully exported: {}", filename))
+            .icon("dialog-information")  // Standard icon name (freedesktop.org)
+            .timeout(5000)  // 5 seconds
+            .show()
+        {
+            eprintln!("⚠️  Could not send desktop notification: {}", e);
+        }
+    });
 }
 
-/// Send a desktop notification for export error
-///
-/// # Arguments
-/// * `error_message` - The error message to display
-///
-/// # Returns
-/// Ok(()) if notification sent successfully, Err with log message if failed
-pub fn notify_export_error(error_message: &str) -> Result<(), String> {
-    // Truncate very long error messages
-    let truncated = if error_message.len() > 100 {
-        format!("{}...", &error_message[..97])
+/// Show a desktop notification for an export error (fire-and-forget).
+pub fn notify_export_error(error_message: &str) {
+    // Truncate very long error messages (on a char boundary).
+    let truncated = if error_message.chars().count() > 100 {
+        let prefix: String = error_message.chars().take(97).collect();
+        format!("{}...", prefix)
     } else {
         error_message.to_string()
     };
 
-    Notification::new()
-        .summary("Export Failed")
-        .body(&truncated)
-        .icon("dialog-error")  // Standard error icon
-        .timeout(10000)  // 10 seconds for errors (longer to read)
-        .show()
-        .map_err(|e| format!("Failed to show notification: {}", e))?;
-
-    Ok(())
+    std::thread::spawn(move || {
+        if let Err(e) = Notification::new()
+            .summary("Export Failed")
+            .body(&truncated)
+            .icon("dialog-error")  // Standard error icon
+            .timeout(10000)  // 10 seconds for errors (longer to read)
+            .show()
+        {
+            eprintln!("⚠️  Could not send desktop notification: {}", e);
+        }
+    });
 }