test(widgets): pin the Tabs behaviour that keepMounted={false} depends on

health-monitoring and media-missing pass keepMounted={false} because Mantine
otherwise builds the inactive panel's whole subtree. That came from reading
Mantine's docs, which is a weak basis for a prop that trades a remount cost for a
mount saving: if Mantine ever stopped rendering inactive panels, the prop would be
pure loss and nobody would notice.

Verified in the installed version instead. With three marker children per panel,
the default renders 3 of 3 children of the *inactive* panel into the DOM, and
keepMounted={false} renders 0. If that changes, this fails and the prop should
come back out.

Also records two things checked while re-reading the audit:

_inputs/ is excluded from the widget sweep on purpose, not by oversight - its
2,748 lines are option editors reached only from the edit modal and the
custom-widget form, and the one widget importing from it uses `import type`,
which is erased at build.

The "statically analyzable dynamic imports" rule has exactly one exception,
translation/src/mapping.ts building import(`./lang/${language}.json`) over 36
languages. It stands: the rule targets accidental over-inclusion, and every
language is genuinely reachable since the user can switch at runtime. Server-side
message loading is already memoised per locale, so it is not per-request either.

packages/widgets/src/common/tabs-keep-mounted.spec.ts

@@ -0,0 +1,102 @@
+import { MantineProvider, Tabs } from "@mantine/core";
+import { act, createElement } from "react";
+import { createRoot } from "react-dom/client";
+import { afterEach, beforeAll, describe, expect, it } from "vitest";
+
+import type { Root } from "react-dom/client";
+
+/**
+ * `health-monitoring` and `media-missing` pass `keepMounted={false}` to `Tabs` because Mantine
+ * otherwise builds the inactive panel's whole subtree - two live monitoring views, or two full lists of
+ * media cards, when only one is ever visible.
+ *
+ * That reasoning came from Mantine's documentation, not from this codebase, and it is the entire
+ * justification for the prop. If a future Mantine version stops rendering inactive panels, the prop
+ * becomes dead weight and the remount cost it buys is being paid for nothing. This pins the behaviour
+ * in the version actually installed.
+ */
+describe("Mantine Tabs keepMounted", () => {
+  let container: HTMLDivElement;
+  let root: Root;
+
+  beforeAll(() => {
+    Object.defineProperty(globalThis, "IS_REACT_ACT_ENVIRONMENT", { value: true, writable: true });
+    Object.defineProperty(window, "matchMedia", {
+      writable: true,
+      value: (query: string) => ({
+        matches: false,
+        media: query,
+        onchange: null,
+        addListener: () => undefined,
+        removeListener: () => undefined,
+        addEventListener: () => undefined,
+        removeEventListener: () => undefined,
+        dispatchEvent: () => false,
+      }),
+    });
+  });
+
+  afterEach(() => {
+    root?.unmount();
+    container?.remove();
+  });
+
+  /** Two panels, each with a countable marker per "row", so hidden work is visible to the assertions. */
+  const renderTabs = async (keepMounted: boolean | undefined) => {
+    container = document.createElement("div");
+    document.body.appendChild(container);
+    root = createRoot(container);
+
+    // `children` goes in the props object rather than as variadic arguments, because Tabs.Panel
+    // declares it as a required prop.
+    const panel = (value: string, marker: string) =>
+      createElement(Tabs.Panel, {
+        value,
+        key: value,
+        children: Array.from({ length: 3 }, (_unused, index) =>
+          createElement("span", { className: marker, key: index }, `${marker}-${index}`),
+        ),
+      });
+
+    await act(async () => {
+      root.render(
+        createElement(
+          MantineProvider,
+          null,
+          createElement(
+            Tabs,
+            { defaultValue: "visible", ...(keepMounted === undefined ? {} : { keepMounted }) },
+            createElement(
+              Tabs.List,
+              null,
+              createElement(Tabs.Tab, { value: "visible" }, "Visible"),
+              createElement(Tabs.Tab, { value: "hidden" }, "Hidden"),
+            ),
+            panel("visible", "shown"),
+            panel("hidden", "offscreen"),
+          ),
+        ),
+      );
+    });
+
+    return {
+      shown: container.querySelectorAll(".shown").length,
+      offscreen: container.querySelectorAll(".offscreen").length,
+    };
+  };
+
+  it("renders the inactive panel's children by default, which is the cost being avoided", async () => {
+    const counts = await renderTabs(undefined);
+
+    expect(counts.shown).toBe(3);
+    // The whole reason the widgets pass keepMounted={false}: this is not 0.
+    expect(counts.offscreen).toBe(3);
+  });
+
+  it("does not render the inactive panel's children when keepMounted is false", async () => {
+    const counts = await renderTabs(false);
+
+    expect(counts.shown).toBe(3);
+    expect(counts.offscreen).toBe(0);
+  });
+});