diff --git a/src/components/App/client-api.ts b/src/components/App/client-api.ts
index f1ccfc2b..b54912ad 100644
--- a/src/components/App/client-api.ts
+++ b/src/components/App/client-api.ts
@@ -2,14 +2,16 @@ import App from './index';
 
 import { expose } from 'comlink';
 
+const API_VERSION = 1;
+
 export function exposeAPI(app: App) {
-  self.parent.postMessage('READY', '*');
+  self.parent.postMessage({ type: 'READY', version: API_VERSION }, '*');
   self.addEventListener('message', (ev: MessageEvent) => {
     if (ev.data !== 'READY?') {
       return;
     }
     ev.stopPropagation();
-    self.parent.postMessage('READY', '*');
+    self.parent.postMessage({ type: 'READY', version: API_VERSION }, '*');
   });
   expose(new API(app), self.parent);
 }
@@ -18,11 +20,13 @@ class API {
   constructor(private app: App) { }
 
   async setFile(blob: Blob, name: string) {
-    await new Promise((resolve) => {
-      this.app.setState({ file: new File([blob], name) }, resolve);
-    });
-    await new Promise((resolve) => {
-      document.addEventListener('squooshingdone', resolve, { once: true });
+    return new Promise(async (resolve) => {
+      document.addEventListener(
+        'squoosh:processingstart',
+        () => resolve(),
+        { once: true },
+      );
+      this.app.openFile(new File([blob], name));
     });
   }
 
@@ -30,8 +34,41 @@ class API {
     if (!this.app.state.file || !this.app.compressInstance) {
       throw new Error('No file has been loaded');
     }
+    if (
+      !this.app.compressInstance!.state.loading &&
+      !this.app.compressInstance!.state.sides[side].loading
+    ) {
+      return this.app.compressInstance!.state.sides[side].file;
+    }
 
-    await this.app.compressInstance.compressionJobs[side];
-    return this.app.compressInstance.state.sides[side].file;
+    return new Promise((resolve, reject) => {
+      document.addEventListener(
+        'squoosh:processingdone',
+        (ev) => {
+          if ((ev as CustomEvent).detail.side !== side) {
+            return;
+          }
+          resolve(this.app.compressInstance!.state.sides[side].file);
+        },
+      );
+      document.addEventListener(
+        'squoosh:processingabort',
+        (ev) => {
+          if ((ev as CustomEvent).detail.side !== side) {
+            return;
+          }
+          reject('aborted');
+        },
+      );
+      document.addEventListener(
+        'squoosh:processingerroor',
+        (ev) => {
+          if ((ev as CustomEvent).detail.side !== side) {
+            return;
+          }
+          reject((ev as CustomEvent).detail.msg);
+        },
+      );
+    });
   }
 }
diff --git a/src/components/App/index.tsx b/src/components/App/index.tsx
index 7d4e3ded..a0703673 100644
--- a/src/components/App/index.tsx
+++ b/src/components/App/index.tsx
@@ -73,6 +73,11 @@ export default class App extends Component<Props, State> {
     import('./client-api').then(m => m.exposeAPI(this));
   }
 
+  @bind openFile(file: File | Fileish) {
+    this.openEditor();
+    this.setState({ file });
+  }
+
   @bind
   private onFileDrop({ files }: FileDropEvent) {
     if (!files || files.length === 0) return;
@@ -83,8 +88,7 @@ export default class App extends Component<Props, State> {
 
   @bind
   private onIntroPickFile(file: File | Fileish) {
-    this.openEditor();
-    this.setState({ file });
+    return this.openFile(file);
   }
 
   @bind
diff --git a/src/components/compress/index.tsx b/src/components/compress/index.tsx
index c84fe7d7..bc470854 100644
--- a/src/components/compress/index.tsx
+++ b/src/components/compress/index.tsx
@@ -205,7 +205,6 @@ const originalDocumentTitle = document.title;
 
 export default class Compress extends Component<Props, State> {
   widthQuery = window.matchMedia('(max-width: 599px)');
-  compressionJobs: [Promise<void>, Promise<void>] = [Promise.resolve(), Promise.resolve()];
 
   state: State = {
     source: undefined,
@@ -306,7 +305,7 @@ export default class Compress extends Component<Props, State> {
       // The image only needs updated if the encoder/preprocessor settings have changed, or the
       // source has changed.
       if (sourceDataChanged || encoderChanged || preprocessorChanged) {
-        this.compressionJobs[i] = this.updateImage(i, {
+        this.queueUpdateImage(i, {
           skipPreprocessing: !sourceDataChanged && !preprocessorChanged,
         });
       }
@@ -396,8 +395,8 @@ export default class Compress extends Component<Props, State> {
     // Either processor is good enough here.
     const processor = this.leftProcessor;
 
-    this.setState({ loadingCounter, loading: true });
-
+    this.setState({ loadingCounter, loading: true }, this.signalProcessingStart);
+    // this.signalProcessingStart();
     // Abort any current encode jobs, as they're redundant now.
     this.leftProcessor.abortCurrent();
     this.rightProcessor.abortCurrent();
@@ -445,7 +444,9 @@ export default class Compress extends Component<Props, State> {
       this.updateDocumentTitle(file.name);
       this.setState(newState);
     } catch (err) {
-      if (err.name === 'AbortError') return;
+      if (err.name === 'AbortError') {
+        return;
+      }
       console.error(err);
       // Another file has been opened/processed before this one processed.
       if (this.state.loadingCounter !== loadingCounter) return;
@@ -467,7 +468,8 @@ export default class Compress extends Component<Props, State> {
 
     this.updateImageTimeoutIds[index] = self.setTimeout(
       () => {
-        this.updateImage(index, options).catch((err) => {
+        this.updateImage(index, options)
+        .catch((err) => {
           console.error(err);
         });
       },
@@ -475,6 +477,31 @@ export default class Compress extends Component<Props, State> {
     );
   }
 
+  @bind
+  private dispatchCustomEvent(name: string, detail?: {}) {
+    (this.base || document).dispatchEvent(new CustomEvent(name, { detail, bubbles: true }));
+  }
+
+  @bind
+  private signalProcessingStart() {
+    this.dispatchCustomEvent('squoosh:processingstart');
+  }
+
+  @bind
+  private signalProcessingDone(side: 0|1) {
+    this.dispatchCustomEvent('squoosh:processingdone', { side });
+  }
+
+  @bind
+  private signalProcessingAbort(side: 0|1) {
+    this.dispatchCustomEvent('squoosh:processingabort', { side });
+  }
+
+  @bind
+  private signalProcessingError(side: 0|1, msg: string) {
+    this.dispatchCustomEvent('squoosh:processingerror', { side, msg });
+  }
+
   private async updateImage(index: number, options: UpdateImageOptions = {}): Promise<void> {
     const {
       skipPreprocessing = false,
@@ -490,9 +517,7 @@ export default class Compress extends Component<Props, State> {
       loading: true,
     });
 
-    this.setState({ sides }, () => this.base!.dispatchEvent(
-      new CustomEvent('squooshingdone', { bubbles: true }),
-    ));
+    this.setState({ sides });
 
     const side = sides[index];
     const settings = side.latestSettings;
@@ -538,8 +563,13 @@ export default class Compress extends Component<Props, State> {
           });
         }
       } catch (err) {
-        if (err.name === 'AbortError') return;
-        this.props.showSnack(`Processing error (type=${settings.encoderState.type}): ${err}`);
+        if (err.name === 'AbortError') {
+          this.signalProcessingAbort(index as 0 | 1);
+          return;
+        }
+        const errorMsg = `Processing error (type=${settings.encoderState.type}): ${err}`;
+        this.signalProcessingError(index as 0|1, errorMsg);
+        this.props.showSnack(errorMsg);
         throw err;
       }
     }
@@ -562,7 +592,7 @@ export default class Compress extends Component<Props, State> {
       encodedSettings: settings,
     });
 
-    this.setState({ sides });
+    this.setState({ sides }, () => this.signalProcessingDone(index as 0|1));
   }
 
   render({ onBack }: Props, { loading, sides, source, mobileView }: State) {