fix(e2e): tighten vitals/frames coverage, retract dead WebKit plan

- drop the vacuous "non-empty method" frames test; the fallback it
  guards makes it unfailable
- vitals spec now asserts values, not just attribute names: finite
  and non-negative for every reported vital, strictly positive for
  ttfb/fcp/lcp (not cls/inp, which can legitimately be zero)
- label the CHROMIUM_ONLY_VITALS pin as tied to today's Playwright
  browser builds, not a client guarantee
- move the WebKit-race retraction to the top of its design doc
  section, reword both headings so they stop asserting a client bug,
  and correct the Goal so it no longer implies CI re-proves this
  automatically
- document hover-preload's effect on SvelteKit traces in the
  package README, not just the internal spec

Author: DriesH

docs/superpowers/specs/2026-08-11-cross-engine-e2e-design.md

@@ -11,8 +11,10 @@ labelled as such.
 
 ## Goal
 
-Prove the JavaScript client behaves correctly on all three browser engines, and keep proving it.
-Tracing is the part that has to be right.
+Prove the JavaScript client behaves correctly on all three browser engines, and make that provable on
+demand. Tracing is the part that has to be right. The engine axis is opt-in and defaults to Chromium;
+wiring it into continuous integration so it re-proves itself automatically is not done (see "Out of
+scope").
 
 ## How this started, and why the instrument changed
 
@@ -53,7 +55,7 @@ traceparent propagation, pageload and navigation roots, span nesting, parameteri
 component trees, span errors, aborted XHR handling and the keepalive flush on unload all work on all
 three engines, in every framework integration.
 
-### The one bug: SvelteKit load fetch nests under the wrong root on WebKit
+### The WebKit failure (not a client bug — retracted)
 
 > **Retracted 2026-08-11.** This is not a client defect. See "Spike result" below: the fetch is a
 > hover-triggered SvelteKit preload fired by Playwright's own pointer movement, before the navigation
@@ -165,7 +167,13 @@ preserved when the projects become generated rather than literal.
 
 Contributors need `npx playwright install firefox webkit` once. Document that next to the new script.
 
-## Part two: fix the WebKit race
+## Part two: the WebKit failure (retracted — see below)
+
+> **Retracted 2026-08-11.** Neither shape below was built. The spike in "Spike result (2026-08-11)"
+> found the premise wrong: `beforeNavigate` is not the earliest signal available, because SvelteKit's
+> hover-preload fires a `load` fetch before any navigation-lifecycle hook runs at all. There is no
+> client bug to fix here; see "The WebKit failure (not a client bug — retracted)" above. The prose
+> below is kept as the record of what was considered before the spike ruled it out.
 
 Chosen direction: **an optional companion for the root layout.**
 

e2e/specs/engines.ts

@@ -4,7 +4,12 @@
 /** Reported by every engine. Measured 2026-08-11 on Chromium, Firefox 150.0.2 and WebKit 26.4. */
 export const UNIVERSAL_VITALS = ['ttfb', 'fcp', 'lcp', 'inp'] as const;
 
-/** Firefox and WebKit implement no layout-shift observer, so CLS never arrives there. */
+/**
+ * Firefox and WebKit implement no layout-shift observer, so CLS never arrives there. This pins the
+ * browser builds Playwright ships today. If a future Playwright Firefox or WebKit build adds a
+ * layout-shift observer, this test starts failing because the engine gained support, not because the
+ * client broke — update this list, don't go hunting for a client bug.
+ */
 export const CHROMIUM_ONLY_VITALS = ['cls'] as const;
 
 export const expectedVitals = (browserName: string): string[] =>

e2e/specs/frames.spec.ts

@@ -43,20 +43,4 @@ test.describe('stack frames', () => {
         // fetched, which is Vite's transformed output, and that is double-quoted today.
         expect(top.codeSnippet[String(top.lineNumber)]).toMatch(/throw new Error\(['"]sync-throw['"]\)/);
     });
-
-    // Every engine has to yield a usable name, whether the engine supplied one or createStackTrace fell
-    // back. An empty method would reach the Flare interface as a blank row.
-    test('every frame carries a non-empty method', async ({ page, fakeFlare }) => {
-        await page.goto('/broken');
-        await page.waitForLoadState('networkidle');
-        await page.getByTestId(testIds.brokenTrigger('sync-throw')).click();
-
-        const report = await fakeFlare.waitForReport({ timeout: 9000, predicate: isSyncThrow });
-        const frames = (report.bodyJson as { stacktrace?: Frame[] }).stacktrace ?? [];
-
-        expect(frames.length).toBeGreaterThan(0);
-        for (const frame of frames) {
-            expect(frame.method.length).toBeGreaterThan(0);
-        }
-    });
 });

e2e/specs/vitals.spec.ts

@@ -4,7 +4,7 @@ import { testIds } from '../../playgrounds/shared/src';
 import type { FakeFlare } from '../fixtures/fake-flare';
 import { expect, test } from '../fixtures/fake-flare';
 import { expectedVitals } from './engines';
-import { attributeKeys, spansOf } from './otlp';
+import { attr, attributeKeys, spansOf } from './otlp';
 
 const PREFIX = 'browser.web_vital.';
 
@@ -23,6 +23,25 @@ const vitalsReported = async (fakeFlare: FakeFlare): Promise<string[]> => {
     return [...names].toSorted();
 };
 
+/** Latest value seen for each reported vital, across every envelope captured so far. */
+const vitalValues = async (fakeFlare: FakeFlare): Promise<Record<string, number>> => {
+    const values: Record<string, number> = {};
+    for (const record of await fakeFlare.traces()) {
+        for (const span of spansOf(record.bodyJson)) {
+            for (const key of attributeKeys(span)) {
+                if (key.startsWith(PREFIX)) {
+                    const value = attr(span, key) as { intValue?: number; doubleValue?: number } | undefined;
+                    const number = value?.intValue ?? value?.doubleValue;
+                    if (typeof number === 'number') {
+                        values[key.slice(PREFIX.length)] = number;
+                    }
+                }
+            }
+        }
+    }
+    return values;
+};
+
 /**
  * A full page life: load, interact, leave. The click has to happen before any navigation or INP is
  * never recorded, and pagehide is what flushes the late vitals span.
@@ -60,5 +79,19 @@ test.describe('web vitals', () => {
         await expect.poll(() => vitalsReported(fakeFlare), { timeout: 9000 }).toEqual(expect.arrayContaining(expected));
 
         expect(await vitalsReported(fakeFlare)).toEqual(expected);
+
+        // This catches a client emitting 0, NaN or a non-number for a reported vital. It does not
+        // catch a unit change (milliseconds vs. seconds), since both are plausible positive numbers.
+        const values = await vitalValues(fakeFlare);
+        for (const name of expected) {
+            const value = values[name];
+            expect(Number.isFinite(value)).toBe(true);
+            expect(value).toBeGreaterThanOrEqual(0);
+        }
+        // Only these three are guaranteed strictly positive for a real page load. cls can be exactly 0
+        // (a perfect score, no layout shift) and inp can legitimately round to 0 for a fast interaction.
+        for (const name of ['ttfb', 'fcp', 'lcp']) {
+            expect(values[name]).toBeGreaterThan(0);
+        }
     });
 });

packages/sveltekit/README.md

@@ -63,6 +63,15 @@ flare.setUser({ id: 123, email: 'jane@example.com', fullName: 'Jane Doe' });
 
 See the [JavaScript identifying-users docs](https://flareapp.io/docs/javascript/data-collection/identifying-users) for the full field list. Pass `null` to clear.
 
+## Hover-preloaded fetches in traces
+
+SvelteKit's recommended default, `data-sveltekit-preload-data="hover"`, starts a route's `load`
+function, including its `fetch`, as soon as a visitor hovers a link — before they click it. If they
+never click, a trace for the page they were on can still show a fetch for the route they hovered but
+never opened. That is a correct record of what the browser did, not a bug, but it can read as wrong
+data in a waterfall. Setting `data-sveltekit-preload-data` to a less eager value (for example `"tap"`)
+avoids it.
+
 ## Documentation
 
 Full documentation on `handleErrorWithFlare`, `captureError`, `trackRouteContext`, lifecycle callbacks, and more is