fix(insights): return values sub-page back-link to the originating metric

The show-all-readings link now carries the originating metric page as a
sanitised from param, so a weight to show-all drill-in returns to weight
rather than the Insights overview. The param is constrained to an internal
/insights/<slug> path so a crafted value cannot redirect off-site.

src/app/insights/values/[type]/page.tsx

@@ -2,10 +2,14 @@
 
 import { use } from "react";
 import Link from "next/link";
-import { notFound } from "next/navigation";
+import { notFound, useSearchParams } from "next/navigation";
 import { ArrowLeft } from "lucide-react";
 
 import { useTranslations } from "@/lib/i18n/context";
+import {
+  INSIGHTS_OVERVIEW,
+  resolveValuesBackHref,
+} from "@/lib/insights/values-back-link";
 import { measurementTypeEnum } from "@/lib/validations/measurement";
 import { MEASUREMENT_TYPE_LABEL_KEYS } from "@/components/measurements/measurement-list-meta";
 import { Button } from "@/components/ui/button";
@@ -33,6 +37,7 @@ export default function InsightsMetricValuesPage({
 }) {
   const { t } = useTranslations();
   const { type } = use(params);
+  const searchParams = useSearchParams();
 
   // Reject any segment that is not a real MeasurementType so a typo'd
   // deep-link 404s instead of mounting a blank list.
@@ -43,6 +48,16 @@ export default function InsightsMetricValuesPage({
   const labelKey = MEASUREMENT_TYPE_LABEL_KEYS[type];
   const metricLabel = labelKey ? t(labelKey) : type;
 
+  // v1.10.2 — return to the originating metric page when the link carried a
+  // `from` param (set by `<SubPageShell>` on the "show all readings" link),
+  // so a `weight → show all values` drill-in lands back on `weight` rather
+  // than the Insights overview. `resolveValuesBackHref` sanitises the value to
+  // an internal `/insights/<slug>` path so a crafted `?from=` can never become
+  // an off-site or protocol-relative target; anything else falls back to the
+  // overview.
+  const backHref = resolveValuesBackHref(searchParams.get("from"));
+  const backToOrigin = backHref !== INSIGHTS_OVERVIEW;
+
   return (
     <SubPageShell
       title={t("insights.subPage.valuesPageTitle", { metric: metricLabel })}
@@ -55,9 +70,11 @@ export default function InsightsMetricValuesPage({
           data-slot="metric-values-back"
           className="-ml-2 w-fit"
         >
-          <Link href="/insights">
+          <Link href={backHref}>
             <ArrowLeft className="mr-1 size-4" aria-hidden="true" />
-            {t("insights.subPage.valuesBack")}
+            {backToOrigin
+              ? t("insights.subPage.valuesBackToMetric", { metric: metricLabel })
+              : t("insights.subPage.valuesBack")}
           </Link>
         </Button>
       }

src/components/insights/sub-page-shell.tsx

@@ -2,6 +2,7 @@
 
 import { useEffect, useRef, type ReactNode } from "react";
 import Link from "next/link";
+import { usePathname } from "next/navigation";
 import { ListOrdered } from "lucide-react";
 
 import { Badge } from "@/components/ui/badge";
@@ -129,6 +130,11 @@ export function SubPageShell({
   children,
 }: SubPageShellProps) {
   const { t } = useTranslations();
+  // v1.10.2 — the "show all readings" link carries the originating metric
+  // page as a `from` param so the values sub-page can offer a back-link to
+  // where the user drilled in from (e.g. `weight → show all values → back to
+  // weight`) rather than always returning to the Insights overview.
+  const pathname = usePathname();
   // a11y: focus the heading on mount so a tab-strip navigation actually
   // moves screen-reader focus into the sub-page body. v1.4.27 MB7 /
   // CF-35 made the focus call opt-in via `focusOnMount` — the
@@ -254,7 +260,11 @@ export function SubPageShell({
           data-slot="metric-show-all-values"
           className="h-10 w-full sm:w-auto"
         >
-          <Link href={`/insights/values/${showAllValuesType}`}>
+          <Link
+            href={`/insights/values/${showAllValuesType}${
+              pathname ? `?from=${encodeURIComponent(pathname)}` : ""
+            }`}
+          >
             <ListOrdered className="mr-1.5 size-4" aria-hidden="true" />
             {t("insights.subPage.showAllValues")}
           </Link>

src/lib/insights/__tests__/values-back-link.test.ts

@@ -0,0 +1,37 @@
+import { describe, it, expect } from "vitest";
+
+import { resolveValuesBackHref } from "../values-back-link";
+
+/**
+ * v1.10.2 — the `/insights/values/[type]` back-link returns to the originating
+ * metric page via a sanitised `from` param. The guard keeps the target an
+ * internal `/insights/<slug>` path so a crafted value can never redirect
+ * off-site.
+ */
+describe("resolveValuesBackHref", () => {
+  it("returns the originating metric page when from is an internal insights path", () => {
+    expect(resolveValuesBackHref("/insights/weight")).toBe("/insights/weight");
+    expect(resolveValuesBackHref("/insights/blood-pressure")).toBe(
+      "/insights/blood-pressure",
+    );
+  });
+
+  it("falls back to the overview when from is missing", () => {
+    expect(resolveValuesBackHref(null)).toBe("/insights");
+    expect(resolveValuesBackHref(undefined)).toBe("/insights");
+    expect(resolveValuesBackHref("")).toBe("/insights");
+  });
+
+  it("rejects the overview itself and any non-metric path", () => {
+    expect(resolveValuesBackHref("/insights")).toBe("/insights");
+    expect(resolveValuesBackHref("/settings/account")).toBe("/insights");
+  });
+
+  it("rejects off-site and protocol-relative targets", () => {
+    expect(resolveValuesBackHref("https://evil.example/insights/weight")).toBe(
+      "/insights",
+    );
+    expect(resolveValuesBackHref("//evil.example")).toBe("/insights");
+    expect(resolveValuesBackHref("/insights//evil.example")).toBe("/insights");
+  });
+});

src/lib/insights/values-back-link.ts

@@ -0,0 +1,25 @@
+/**
+ * v1.10.2 — resolve the back-link target for `/insights/values/[type]`.
+ *
+ * The "show all readings" link (built by `<SubPageShell>`) carries the
+ * originating metric page as a `from` query param so the values sub-page can
+ * return the user to where they drilled in from (e.g. `weight → show all
+ * values → back to weight`) rather than always to the Insights overview.
+ *
+ * The param is sanitised to an internal `/insights/<slug>` path — a single
+ * leading slash, no protocol / host, no protocol-relative `//` — so a crafted
+ * `?from=` value can never become an off-site or protocol-relative navigation
+ * target. Anything that fails the check falls back to the Insights overview.
+ */
+export const INSIGHTS_OVERVIEW = "/insights";
+
+export function resolveValuesBackHref(from: string | null | undefined): string {
+  if (
+    from &&
+    from.startsWith("/insights/") &&
+    !from.startsWith("/insights//")
+  ) {
+    return from;
+  }
+  return INSIGHTS_OVERVIEW;
+}