Metrics: Tag CLS elements

This will help people dig into pages with bad layout shifts

Co-authored-by: Alberto Leal <mail4alberto@gmail.com>

Author: jarstelfox

packages/tracing/src/browser/metrics.ts

@@ -6,7 +6,7 @@ import { browserPerformanceTimeOrigin, getGlobalObject, htmlTreeAsString, isNode
 import { Span } from '../span';
 import { Transaction } from '../transaction';
 import { msToSec } from '../utils';
-import { getCLS } from './web-vitals/getCLS';
+import { getCLS, LayoutShift } from './web-vitals/getCLS';
 import { getFID } from './web-vitals/getFID';
 import { getLCP, LargestContentfulPaint } from './web-vitals/getLCP';
 import { getFirstHidden } from './web-vitals/lib/getFirstHidden';
@@ -20,6 +20,7 @@ export class MetricsInstrumentation {
 
   private _performanceCursor: number = 0;
   private _lcpEntry: LargestContentfulPaint | undefined;
+  private _clsEntry: LayoutShift | undefined;
 
   public constructor() {
     if (!isNodeEnv() && global?.performance) {
@@ -207,6 +208,13 @@ export class MetricsInstrumentation {
 
         transaction.setTag('lcp.size', this._lcpEntry.size);
       }
+
+      if (this._clsEntry) {
+        logger.log('[Measurements] Adding CLS Data');
+        transaction.setData('measurements.cls', {
+          sources: this._clsEntry.sources.map(source => htmlTreeAsString(source.node)),
+        });
+      }
     }
   }
 
@@ -221,6 +229,7 @@ export class MetricsInstrumentation {
 
       logger.log('[Measurements] Adding CLS');
       this._measurements['cls'] = { value: metric.value };
+      this._clsEntry = entry as LayoutShift;
     });
   }
 

packages/tracing/src/browser/web-vitals/getCLS.ts

@@ -21,9 +21,17 @@ import { onHidden } from './lib/onHidden';
 import { ReportHandler } from './types';
 
 // https://wicg.github.io/layout-instability/#sec-layout-shift
-interface LayoutShift extends PerformanceEntry {
+export interface LayoutShift extends PerformanceEntry {
   value: number;
   hadRecentInput: boolean;
+  sources: Array<LayoutShiftAttribution>;
+  toJSON(): Record<string, unknown>;
+}
+
+export interface LayoutShiftAttribution {
+  node?: Node;
+  previousRect: DOMRectReadOnly;
+  currentRect: DOMRectReadOnly;
 }
 
 export const getCLS = (onReport: ReportHandler, reportAllChanges = false): void => {