fix(app): make session navigation stable and fast (#33569)

Co-authored-by: Brendan Allan <git@brendonovich.dev>

packages/app/e2e/performance/timeline/first-navigation-metrics.ts

@@ -0,0 +1,34 @@
+export type FirstNavigationSample = {
+  observedAtMs: number
+  source: boolean
+  destination: boolean
+  content: boolean
+  pathname?: string
+  center?: string
+}
+
+function category(sample: FirstNavigationSample) {
+  if (sample.destination && !sample.source) return "destination"
+  if (sample.source && !sample.destination) return "source"
+  if (!sample.content) return "blank"
+  return "unknown"
+}
+
+export function summarizeFirstNavigation(samples: FirstNavigationSample[]) {
+  const categories = samples.map(category)
+  const stable = categories.findIndex(
+    (value, index) =>
+      value === "destination" &&
+      categories[index + 1] === "destination" &&
+      categories[index + 2] === "destination",
+  )
+  return {
+    samples: samples.length,
+    firstDestinationObservedMs: samples[categories.indexOf("destination")]?.observedAtMs ?? null,
+    stableDestinationObservedMs: stable === -1 ? null : samples[stable + 2]!.observedAtMs,
+    sourceSamples: categories.filter((value) => value === "source").length,
+    blankSamples: categories.filter((value) => value === "blank").length,
+    unknownSamples: categories.filter((value) => value === "unknown").length,
+    destinationSamples: categories.filter((value) => value === "destination").length,
+  }
+}