fix: always use navStart as speedline timeOrigin

lighthouse-core/gather/computed/speedline.js

@@ -28,13 +28,14 @@ class Speedline extends ComputedArtifact {
   /**
    * @return {!Promise}
    */
-  compute_(trace) {
+  compute_(trace, computedArtifacts) {
     // speedline() may throw without a promise, so we resolve immediately
     // to get in a promise chain.
-    return Promise.resolve().then(_ => {
-      return speedline(trace.traceEvents);
-    }).then(speedlineResults => {
-      return speedlineResults;
+    return computedArtifacts.requestTraceOfTab(trace).then(traceOfTab => {
+      // Force use of nav start as reference point for speedline
+      // See https://github.com/GoogleChrome/lighthouse/issues/2095
+      const navStart = traceOfTab.timestamps.navigationStart * 1000;
+      return speedline(trace.traceEvents, {timeOrigin: navStart});
     });
   }
 }

lighthouse-core/test/fixtures/traces/threeframes-blank_content_more.json

@@ -15,6 +15,36 @@
 				"tdur": 186,
 				"tts": 4516530
 			},
+			{
+				"pid": 93449,
+				"tid": 1295,
+				"ts": 900000000000,
+				"ph": "X",
+				"cat": "toplevel",
+				"name": "TracingStartedInPage",
+				"args": {
+					"data": {
+						"page": "dummy"
+					}
+				},
+				"dur": 189,
+				"tdur": 186,
+				"tts": 4516530
+			},
+			{
+				"pid": 93449,
+				"tid": 1295,
+				"ts": 900000000000,
+				"ph": "X",
+				"cat": "loading",
+				"name": "navigationStart",
+				"args": {
+					"frame": "dummy"
+				},
+				"dur": 189,
+				"tdur": 186,
+				"tts": 4516530
+			},
 			{
 				"pid": 93267,
 				"tid": 1295,

lighthouse-core/test/gather/computed/speedline-test.js

@@ -17,16 +17,20 @@
 
 /* eslint-env mocha */
 
-const SpeedlineGather = require('../../../gather/computed/speedline.js');
 const assert = require('assert');
 const pwaTrace = require('../../fixtures/traces/progressive-app.json');
 const threeFrameTrace = require('../../fixtures/traces/threeframes-blank_content_more.json');
+const Runner = require('../../../runner.js');
 
 describe('Speedline gatherer', () => {
-  it('returns an error debugString on faulty trace data', () => {
-    const speedlineGather = new SpeedlineGather();
+  let computedArtifacts;
+
+  beforeEach(() => {
+    computedArtifacts = Runner.instantiateComputedArtifacts();
+  });
 
-    return speedlineGather.request({traceEvents: {boo: 'ya'}}).then(_ => {
+  it('returns an error debugString on faulty trace data', () => {
+    return computedArtifacts.requestSpeedline({traceEvents: {boo: 'ya'}}).then(_ => {
       assert.fail(true, true, 'Invalid trace did not throw exception in speedline');
     }, err => {
       assert.ok(err);
@@ -35,41 +39,36 @@ describe('Speedline gatherer', () => {
   });
 
   it('measures the pwa.rocks example with speed index of 577', () => {
-    const speedlineGather = new SpeedlineGather();
-
-    return speedlineGather.request({traceEvents: pwaTrace}).then(speedline => {
-      return assert.equal(Math.floor(speedline.speedIndex), 577);
+    return computedArtifacts.requestSpeedline({traceEvents: pwaTrace}).then(speedline => {
+      return assert.equal(Math.floor(speedline.speedIndex), 561);
     });
   });
 
   it('measures SI of 3 frame trace (blank @1s, content @2s, more content @3s)', () => {
-    const speedlineGather = new SpeedlineGather();
-
-    return speedlineGather.request({traceEvents: threeFrameTrace}).then(speedline => {
+    return computedArtifacts.requestSpeedline(threeFrameTrace).then(speedline => {
       assert.equal(Math.floor(speedline.speedIndex), 2040);
       return assert.equal(Math.floor(speedline.perceptualSpeedIndex), 2030);
     });
   });
 
   it('uses a cache', () => {
-    const speedlineGather = new SpeedlineGather();
     let start;
+    let firstResult;
     const trace = {traceEvents: pwaTrace};
     // repeat with the same input data twice
     return Promise.resolve()
-      .then(_ => speedlineGather.request(trace))
-      .then(_ => {
+      .then(_ => computedArtifacts.requestSpeedline(trace))
+      .then(result => {
         start = Date.now();
+        firstResult = result;
       })
-      .then(_ => speedlineGather.request(trace))
+      .then(_ => computedArtifacts.requestSpeedline(trace))
       .then(speedline => {
         // on a MacBook Air, one run is  1000-1500ms
         assert.ok(Date.now() - start < 50, 'Quick results come from the cache');
+        assert.equal(firstResult, speedline, 'Cache match matches');
 
-        assert.ok(speedlineGather._cache.has(trace), 'Cache reports a match');
-        assert.equal(speedlineGather._cache.get(trace), speedline, 'Cache match matches');
-
-        return assert.equal(Math.floor(speedline.speedIndex), 577);
+        return assert.equal(Math.floor(speedline.speedIndex), 561);
       });
   });
 });