diff --git a/lighthouse-core/report/html/renderer/report-ui-features.js b/lighthouse-core/report/html/renderer/report-ui-features.js
index 14ffbd3355bf..a1757ca70632 100644
--- a/lighthouse-core/report/html/renderer/report-ui-features.js
+++ b/lighthouse-core/report/html/renderer/report-ui-features.js
@@ -353,25 +353,19 @@ class ReportUIFeatures {
    * @protected
    */
   static openTabAndSendJsonReport(reportJson, viewerPath) {
-    let resolve;
-    const p = new Promise(res => resolve = res);
     // Chrome doesn't allow us to immediately postMessage to a popup right
     // after it's created. Normally, we could also listen for the popup window's
     // load event, however it is cross-domain and won't fire. Instead, listen
     // for a message from the target app saying "I'm open".
     const json = reportJson;
     window.addEventListener('message', function msgHandler(/** @type {!Event} */ e) {
-      const messageEvent = /** @type {!MessageEvent<{opened: boolean, rendered: boolean}>} */ (e);
+      const messageEvent = /** @type {!MessageEvent<{opened: boolean}>} */ (e);
       if (messageEvent.origin !== VIEWER_ORIGIN) {
         return;
       }
-      // Most recent deployment
       if (messageEvent.data.opened) {
         popup.postMessage({lhresults: json}, VIEWER_ORIGIN);
-      }
-      if (messageEvent.data.rendered) {
         window.removeEventListener('message', msgHandler);
-        resolve(popup);
       }
     });
 
@@ -379,8 +373,6 @@ class ReportUIFeatures {
     const fetchTime = json.fetchTime || json.generatedTime;
     const windowName = `${json.lighthouseVersion}-${json.requestedUrl}-${fetchTime}`;
     const popup = /** @type {!Window} */ (window.open(`${VIEWER_ORIGIN}${viewerPath}`, windowName));
-
-    return p;
   }
 
   /**
diff --git a/lighthouse-viewer/app/src/lighthouse-report-viewer.js b/lighthouse-viewer/app/src/lighthouse-report-viewer.js
index a67246456833..0dac51d60938 100644
--- a/lighthouse-viewer/app/src/lighthouse-report-viewer.js
+++ b/lighthouse-viewer/app/src/lighthouse-report-viewer.js
@@ -175,7 +175,7 @@ class LighthouseReportViewer {
   }
 
   /**
-   * Opens new tab with compatible report viewer version
+   * Opens legacy viewer in current tab, then renders report
    * @param {!ReportRenderer.ReportJSON} reportJson
    * @private
    */
@@ -183,6 +183,7 @@ class LighthouseReportViewer {
     const warnMsg = `Version mismatch between viewer and JSON. Opening compatible viewer...`;
     logger.log(warnMsg, false);
 
+    // Place report in IDB, then navigate current tab to the legacy viewer
     const viewerPath = new URL('../viewer2x/', location.href);
     idbKeyval.set('2xreport', json).then(_ => {
       window.location.href = viewerPath;
diff --git a/lighthouse-viewer/app/src/viewer-ui-features.js b/lighthouse-viewer/app/src/viewer-ui-features.js
index e213bf358d32..8d880b157d2a 100644
--- a/lighthouse-viewer/app/src/viewer-ui-features.js
+++ b/lighthouse-viewer/app/src/viewer-ui-features.js
@@ -46,6 +46,13 @@ class ViewerUIFeatures extends ReportUIFeatures {
     return ReportGenerator.generateReportHtml(this.json);
   }
 
+  /**
+   * @override
+   */
+  sendJsonReport() {
+    throw new Error('Cannot send JSON to Viewer from Viewer.');
+  }
+
   /**
    * @override
    */