commit 8638969b382dab15e06b9a9690d268fff26414fa
Author: Ole <oler@google.com>
Date:   Fri Oct 9 20:43:54 2020 +0200

    Refactor reload 9: Extract promisification
    
    This function is very hard to follow. Making small steps to ensure
    careful review.
    
    Making this function a bit shorter and extracting this unrelated event
    promisification.
    
    Change-Id: Ib7194c36dfcdf9ca2960b25d49e48785f0d082e6

diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
index 52bd560..61db314 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
@@ -357,20 +357,13 @@ export class GrDiffHost extends GestureEventListeners(
 
       this.filesWeblinks = this._getFilesWeblinks(diff);
       this.diff = diff;
-      const event = await new Promise<CustomEvent>(resolve => {
-        const callback = (event: CustomEvent) => {
-          this.removeEventListener('render', callback);
-          resolve(event);
-        };
-        this.addEventListener('render', callback);
-      });
+      const event = await this._onRenderOnce();
       if (shouldReportMetric) {
         // We report diffViewContentDisplayed only on reload caused
         // by params changed - expected only on Diff Page.
         this.reporting.diffViewContentDisplayed();
       }
-      const needsSyntaxHighlighting =
-        event.detail && event.detail.contentRendered;
+      const needsSyntaxHighlighting = !!event.detail?.contentRendered;
       if (needsSyntaxHighlighting) {
         this.reporting.time(TimingLabel.SYNTAX);
         try {
@@ -396,6 +389,16 @@ export class GrDiffHost extends GestureEventListeners(
     return [this.$.syntaxLayer, ...this.$.jsAPI.getDiffLayers(path, changeNum)];
   }
 
+  private _onRenderOnce(): Promise<CustomEvent> {
+    return new Promise<CustomEvent>(resolve => {
+      const callback = (event: CustomEvent) => {
+        this.removeEventListener('render', callback);
+        resolve(event);
+      };
+      this.addEventListener('render', callback);
+    });
+  }
+
   clear() {
     if (this.path) this.$.jsAPI.disposeDiffLayers(this.path);
     this._layers = [];