add flag to FMP to indicate if it fell back to FMP candidate (#2851)

lighthouse-core/audits/first-meaningful-paint.js

@@ -82,7 +82,8 @@ class FirstMeaningfulPaint extends Audit {
         fMP: traceOfTab.timings.firstMeaningfulPaint,
         onLoad: traceOfTab.timings.onLoad,
         endOfTrace: traceOfTab.timings.traceEnd,
-      }
+      },
+      fmpFellBack: traceOfTab.fmpFellBack
     };
 
     Object.keys(extendedInfo.timings).forEach(key => {

lighthouse-core/closure/typedefs/ComputedArtifacts.js

@@ -34,6 +34,7 @@ let TraceTimes;
     firstContentfulPaintEvt: TraceEvent,
     firstMeaningfulPaintEvt: TraceEvent,
     onLoadEvt: TraceEvent,
+    fmpFellBack: boolean,
   }} */
 let TraceOfTabArtifact;
 

lighthouse-core/gather/computed/trace-of-tab.js

@@ -69,13 +69,15 @@ class TraceOfTab extends ComputedArtifact {
     let firstMeaningfulPaint = frameEvents.find(
       e => e.name === 'firstMeaningfulPaint' && e.ts > navigationStart.ts
     );
+    let fmpFellBack = false;
 
     // If there was no firstMeaningfulPaint event found in the trace, the network idle detection
     // may have not been triggered before Lighthouse finished tracing.
     // In this case, we'll use the last firstMeaningfulPaintCandidate we can find.
     // However, if no candidates were found (a bogus trace, likely), we fail.
     if (!firstMeaningfulPaint) {
       const fmpCand = 'firstMeaningfulPaintCandidate';
+      fmpFellBack = true;
       log.verbose('trace-of-tab', `No firstMeaningfulPaint found, falling back to last ${fmpCand}`);
       const lastCandidate = frameEvents.filter(e => e.name === fmpCand).pop();
       if (!lastCandidate) {
@@ -131,6 +133,7 @@ class TraceOfTab extends ComputedArtifact {
       firstContentfulPaintEvt: firstContentfulPaint,
       firstMeaningfulPaintEvt: firstMeaningfulPaint,
       onLoadEvt: onLoad,
+      fmpFellBack,
     };
   }
 }

lighthouse-core/test/audits/first-meaningful-paint-test.js

@@ -59,6 +59,10 @@ describe('Performance: first-meaningful-paint audit', () => {
     it('scores the fMP correctly', () => {
       assert.equal(fmpResult.score, 99);
     });
+
+    it('did not fall back to an FMP candidate event', () => {
+      assert.ok(!fmpResult.extendedInfo.value.fmpFellBack);
+    });
   });
 
   describe('finds correct FMP', () => {
@@ -68,6 +72,7 @@ describe('Performance: first-meaningful-paint audit', () => {
         assert.equal(result.rawValue, 529.9);
         assert.equal(result.extendedInfo.value.timestamps.navStart, 29343540951);
         assert.equal(result.extendedInfo.value.timings.fCP, 80.054);
+        assert.ok(!result.extendedInfo.value.fmpFellBack);
         assert.ok(!result.debugString);
       });
     });
@@ -78,6 +83,7 @@ describe('Performance: first-meaningful-paint audit', () => {
         assert.equal(result.rawValue, 632.4);
         assert.equal(result.extendedInfo.value.timestamps.navStart, 8885424467);
         assert.equal(result.extendedInfo.value.timings.fCP, 632.419);
+        assert.ok(!result.extendedInfo.value.fmpFellBack);
         assert.ok(!result.debugString);
       });
     });
@@ -88,6 +94,7 @@ describe('Performance: first-meaningful-paint audit', () => {
         assert.equal(result.rawValue, 878.4);
         assert.equal(result.extendedInfo.value.timestamps.navStart, 1805796384607);
         assert.equal(result.extendedInfo.value.timings.fCP, 879.046);
+        assert.ok(!result.extendedInfo.value.fmpFellBack);
         assert.ok(!result.debugString);
       });
     });
@@ -97,6 +104,7 @@ describe('Performance: first-meaningful-paint audit', () => {
         assert.equal(result.displayValue, '4,460\xa0ms');
         assert.equal(result.rawValue, 4460.9);
         assert.equal(result.extendedInfo.value.timings.fCP, 1494.73);
+        assert.ok(result.extendedInfo.value.fmpFellBack);
         assert.ok(!result.debugString);
       });
     });
@@ -111,6 +119,8 @@ describe('Performance: first-meaningful-paint audit', () => {
       assert.strictEqual(result.extendedInfo.value.timings.fMP, 482.318);
       assert.strictEqual(result.extendedInfo.value.timestamps.fCP, undefined);
       assert.strictEqual(result.extendedInfo.value.timestamps.fMP, 2149509604903);
+      // NOTE: falls back to candidate FMP
+      assert.ok(result.extendedInfo.value.fmpFellBack);
     });
   });
 });

lighthouse-core/test/gather/computed/trace-of-tab-test.js

@@ -58,6 +58,7 @@ describe('Trace of Tab computed artifact:', () => {
       assert.equal(trace.navigationStartEvt.ts, 29343540951);
       assert.equal(trace.firstContentfulPaintEvt.ts, 29343621005);
       assert.equal(trace.firstMeaningfulPaintEvt.ts, 29344070867);
+      assert.ok(!trace.fmpFellBack);
     });
 
     it('if there was a tracingStartedInPage after the frame\'s navStart #2', () => {
@@ -66,6 +67,7 @@ describe('Trace of Tab computed artifact:', () => {
       assert.equal(trace.navigationStartEvt.ts, 8885424467);
       assert.equal(trace.firstContentfulPaintEvt.ts, 8886056886);
       assert.equal(trace.firstMeaningfulPaintEvt.ts, 8886056891);
+      assert.ok(!trace.fmpFellBack);
     });
 
     it('if it appears slightly before the fCP', () => {
@@ -74,6 +76,7 @@ describe('Trace of Tab computed artifact:', () => {
       assert.equal(trace.navigationStartEvt.ts, 1805796384607);
       assert.equal(trace.firstContentfulPaintEvt.ts, 1805797263653);
       assert.equal(trace.firstMeaningfulPaintEvt.ts, 1805797262960);
+      assert.ok(!trace.fmpFellBack);
     });
 
     it('from candidates if no defined FMP exists', () => {
@@ -82,6 +85,7 @@ describe('Trace of Tab computed artifact:', () => {
       assert.equal(trace.navigationStartEvt.ts, 2146735807738);
       assert.equal(trace.firstContentfulPaintEvt.ts, 2146737302468);
       assert.equal(trace.firstMeaningfulPaintEvt.ts, 2146740268666);
+      assert.ok(trace.fmpFellBack);
     });
   });
 
@@ -91,6 +95,7 @@ describe('Trace of Tab computed artifact:', () => {
     assert.equal(trace.navigationStartEvt.ts, 2149509122585, 'bad navStart');
     assert.equal(trace.firstContentfulPaintEvt, undefined, 'bad fcp');
     assert.equal(trace.firstMeaningfulPaintEvt.ts, 2149509604903, 'bad fmp');
+    assert.ok(trace.fmpFellBack);
   });
 
   it('handles traces missing a paints (captured in background tab)', () => {