diff --git a/replay-ui/src/__tests__/run-history.test.ts b/replay-ui/src/__tests__/run-history.test.ts
new file mode 100644
index 0000000..5a678c7
--- /dev/null
+++ b/replay-ui/src/__tests__/run-history.test.ts
@@ -0,0 +1,94 @@
+import { describe, it, expect } from "bun:test";
+import {
+  buildRunHistory,
+  collectPropertyNames,
+  sortLanes,
+  statusForProperty,
+} from "../lib/run-history";
+import type { PropertyLane } from "../panels/Timeline";
+import type { Run, Step, StepSummary } from "../types";
+
+function step(over: Partial<Step>): Step {
+  return { step: 0, timestamp: "1970-01-01T00:00:00.000Z", ...over };
+}
+
+function summary(over: Partial<StepSummary>): StepSummary {
+  return {
+    index: 0,
+    timestamp: "1970-01-01T00:00:00.000Z",
+    has_violations: false,
+    has_exceptions: false,
+    ...over,
+  };
+}
+
+function lane(name: string, statuses: PropertyLane["statuses"]): PropertyLane {
+  return { name, statuses };
+}
+
+describe("collectPropertyNames", () => {
+  it("dedups and sorts names across steps, skipping null and residual-less steps", () => {
+    const names = collectPropertyNames([
+      null,
+      step({ residuals: { b: { op: "true" }, a: { op: "false" } } }),
+      step({}),
+      step({ residuals: { a: { op: "true" }, c: { op: "true" } } }),
+    ]);
+    expect(names).toEqual(["a", "b", "c"]);
+  });
+});
+
+// Bug class: getting status precedence wrong (e.g. checking residual before
+// the violation set, or not defaulting null/absent to pending) would paint a
+// violated property lane green.
+describe("statusForProperty", () => {
+  it("ranks violated over a holding residual and defaults to pending", () => {
+    const s = step({ violations: ["p"], residuals: { p: { op: "true" } } });
+    expect(statusForProperty("p", s)).toBe("violated");
+    expect(statusForProperty("q", step({ residuals: { q: { op: "true" } } }))).toBe("holds");
+    expect(statusForProperty("q", step({ residuals: { q: { op: "false" } } }))).toBe("pending");
+    expect(statusForProperty("p", null)).toBe("pending");
+  });
+});
+
+// Bug class: a lane that ever violated must sort first; trailing-pending lanes
+// rank ahead of fully-holding ones, else the timeline buries active failures.
+describe("sortLanes", () => {
+  it("orders violated, then trailing-pending, then holds, ties by name", () => {
+    const ordered = sortLanes([
+      lane("holds-b", ["holds", "holds"]),
+      lane("pending-a", ["holds", "pending"]),
+      lane("violated-z", ["holds", "violated", "holds"]),
+      lane("holds-a", ["holds", "holds"]),
+    ]).map((l) => l.name);
+    expect(ordered).toEqual(["violated-z", "pending-a", "holds-a", "holds-b"]);
+  });
+});
+
+describe("buildRunHistory", () => {
+  it("aligns lane statuses, metrics samples, and first-violation index by position", () => {
+    const run = {
+      id: "run-1",
+      steps: [
+        summary({ index: 0 }),
+        summary({ index: 1, has_violations: true }),
+        summary({ index: 2, has_exceptions: true }),
+      ],
+    } as unknown as Run;
+    const responses = [
+      step({ step: 0, residuals: { p: { op: "true" } } }),
+      step({ step: 1, violations: ["p"], residuals: { p: { op: "true" } } }),
+      null,
+    ];
+
+    const history = buildRunHistory(run, responses);
+
+    expect(history.names).toEqual(["p"]);
+    expect(history.lanes[0].statuses).toEqual(["holds", "violated", "pending"]);
+    expect(history.firstViolationStep).toBe(1);
+    expect(history.firstExceptionStep).toBe(2);
+    expect(history.violationStepIndices).toEqual([1]);
+    expect(history.exceptionStepIndices).toEqual([2]);
+    expect(history.metricsSamples.map((m) => m.stepIndex)).toEqual([0, 1, 2]);
+  });
+});