set `renderSideBySide` when creating diff instance

app/assets/javascripts/ide/lib/editor.js

@@ -69,6 +69,7 @@ export default class Editor {
           occurrencesHighlight: false,
           renderLineHighlight: 'none',
           hideCursorInOverviewRuler: true,
+          renderSideBySide: this.renderSideBySide(domElement),
         })),
       );
 
@@ -177,11 +178,15 @@ export default class Editor {
     if (!this.isDiffEditorType) return;
 
     this.instance.updateOptions({
-      renderSideBySide: this.instance.getDomNode().offsetWidth >= 700,
+      renderSideBySide: this.renderSideBySide(this.instance.getDomNode()),
     });
   }
 
   get isDiffEditorType() {
     return this.instance.getEditorType() === 'vs.editor.IDiffEditor';
   }
+
+  renderSideBySide(domElement) {
+    return domElement.offsetWidth >= 700;
+  }
 }

spec/javascripts/ide/lib/editor_spec.js

@@ -77,6 +77,7 @@ describe('Multi-file editor library', () => {
         renderLineHighlight: 'none',
         hideCursorInOverviewRuler: true,
         wordWrap: 'on',
+        renderSideBySide: true,
       });
     });
   });