diff --git a/index.html b/index.html
index c31321e..9ccea7d 100644
--- a/index.html
+++ b/index.html
@@ -226,7 +226,13 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
         O.onchange = updateConditionals;
         updateConditionals();
-  
+
+        const progressTarget = new EventTarget();
+        progressTarget.addEventListener('data-progress', (e) => console.log(e.detail));
+        // progressTarget.addEventListener('asset-fetch', (e) => console.log(e.detail));
+        // progressTarget.addEventListener('asset-fetched', (e) => console.log(e.detail));
+        progressTarget.addEventListener('nwjs-progress', (e) => console.log(e.detail));
+
         const loadProject = async () => {
           if (O.project.source === 'file') {
             if (!O.project.file) {
@@ -239,7 +245,9 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
             if (!match) {
               throw new Error('Invalid project ID');
             }
-            const project = await SBDownloader.download(match[0]);
+            const project = await SBDownloader.download(match[0], {
+              progressTarget
+            });
             return new Packager.Project(await project.asBlob(), project.type);
           }
           throw new Error('unknown project source');
@@ -269,16 +277,13 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
           switch (O.environment.target) {
             case 'html': return new Packager.environments.HTML();
             case 'zip': return new Packager.environments.Zip();
-            case 'nwjs-win64': return new Packager.environments.NWjs({
-              icon: O.nwjs.icon || getDefaultIcon(),
-              manifest: O.nwjs.manifest,
-              platform: 'windows',
-            });
-            case 'nwjs-mac': return new Packager.environments.NWjs({
-              icon: O.nwjs.icon || getDefaultIcon(),
-              manifest: O.nwjs.manifest,
-              platform: 'mac',
-            })
+            case 'nwjs-win64':
+            case 'nwjs-mac':
+              return new Packager.environments.NWjs({
+                icon: O.nwjs.icon || getDefaultIcon(),
+                manifest: O.nwjs.manifest,
+                platform: O.environment.target.substr('nwjs-'.length),
+              });
           }
           throw new Error('unknown environment');
         };
@@ -313,7 +318,7 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
             const runtime = getRuntime();
             const environment = getEnvironment();
             const projectData = await loadProject();
-            const result = await environment.package(runtime, projectData);
+            const result = await environment.package(runtime, projectData, progressTarget);
             if (!(result.data instanceof Blob)) {
               result.data = new Blob([result.data]);
             }
diff --git a/packager.js b/packager.js
index 83de3c8..b5b2eb1 100644
--- a/packager.js
+++ b/packager.js
@@ -19,14 +19,14 @@ window.Packager = (function() {
   const readAsDataURL = (blob) => new Promise((resolve, reject) => {
     const fr = new FileReader();
     fr.onload = () => resolve(fr.result);
-    fr.onerror = () => reject('could not read as data url');
+    fr.onerror = () => reject(new Error('could not read as data url'));
     fr.readAsDataURL(blob);
   });
 
   const readAsArrayBuffer = (blob) => new Promise((resolve, reject) => {
     const fr = new FileReader();
     fr.onload = () => resolve(fr.result);
-    fr.onerror = () => reject('could not read as arraybuffer');
+    fr.onerror = () => reject(new Error('could not read as arraybuffer'));
     fr.readAsArrayBuffer(blob);
   });
 
@@ -37,18 +37,53 @@ window.Packager = (function() {
     image.src = src;
   });
 
-  const isObject = (obj) => typeof obj === 'object' && obj !== null;
-
   const canvasToBlob = (canvas) => new Promise((resolve, reject) => {
     canvas.toBlob((blob) => {
       if (blob) {
         resolve(blob);
       } else {
-        reject('Could not read <canvas> as blob');
+        reject(new Error('Could not read <canvas> as blob'));
       }
     }); // png is default type, quality is ignored
   });
 
+  const isObject = (obj) => typeof obj === 'object' && obj !== null;
+
+  // TODO: load dynamically
+  const assetManifest = {
+    'nwjs-win64': {
+      src: 'https://packagerdata.turbowarp.org/nwjs-v0.49.0-win-x64.zip',
+      size: 97406745
+    },
+    'nwjs-mac': {
+      src: 'https://packagerdata.turbowarp.org/nwjs-v0.49.0-osx-x64.zip',
+      size: 128684835
+    }
+  };
+
+  const fetchManifestAsset = (assetName, progressCallback) => {
+    const manifestEntry = assetManifest[assetName];
+    if (manifestEntry._data) {
+      return Promise.resolve(manifestEntry._data);
+    }
+    return new Promise((resolve, reject) => {
+      const {src, size} = manifestEntry;
+      const xhr = new XMLHttpRequest();
+      xhr.responseType = 'blob';
+      xhr.onload = () => {
+        manifestEntry._data = xhr.response;
+        resolve(xhr.response);
+      };
+      xhr.onerror = () => reject(new Error('XHR failed'));
+      xhr.onprogress = (e) => {
+        progressCallback(e.loaded / size);
+      };
+      xhr.onloadend = () => progressCallback(1);
+      xhr.open('GET', src);
+      xhr.send();
+    });
+  };
+
   class Project {
     constructor(blob, type) {
       this.blob = blob;
@@ -588,11 +623,14 @@ ${scripts}
       this.platform = platform;
       this.icon = icon;
     }
-    async package(runtime, projectData) {
+    async package(runtime, projectData, progressTarget) {
       const packagerData = await runtime.package(await projectData.asFetchedFrom('project.zip'));
 
-      const response = await fetch(`https://packagerdata.turbowarp.org/nwjs-v0.49.0-${this.platform === 'windows' ? 'win' : 'osx'}-x64.zip`);
-      const nwjsData = await response.arrayBuffer();
+      const nwjsData = await fetchManifestAsset(`nwjs-${this.platform}`, (progress) => {
+        progressTarget.dispatchEvent(new CustomEvent('nwjs-progress', {
+          detail: progress
+        }));
+      })
       const nwjsZip = await JSZip.loadAsync(nwjsData);
 
       // Inside the nw.js zip is another folder called something like nwjs-v0.48.2-win-x64