fix(labs): keep what is typed in the search field when the URL catches up

The labs search writes its trimmed query to the URL after a short pause,
and every change of the URL query replaced the field's text. The
field's own write came back as well: a trailing space vanished, so
"vitamin " followed by "d" searched for "vitamind", and a letter
typed while the write landed was lost. A query the field wrote itself
now leaves the field alone; one from back/forward, a link or Reset
still replaces it.

src/components/labs/__tests__/lab-list-url-sync.browser.test.tsx

@@ -289,6 +289,47 @@ describe("LabList stale category and group header handling", () => {
     expect(readStoredOpen("collapsed")).toEqual({});
   });
 
+  it("keeps a trailing space in the search field after the URL takes the query", async () => {
+    setSearch("");
+    const queryClient = labQueryClient();
+    queryClient.setQueryData(
+      queryKeys.labResultsOverview("en"),
+      overviewWithAbnormalMarker,
+    );
+
+    const { screen } = await renderWithApp(<LabList />, { queryClient });
+
+    const search = screen.getByRole("searchbox", { name: "Search biomarkers" });
+    await search.fill("iron ");
+    await expect.poll(currentSearch).toBe("?q=iron");
+    // One more frame for the URL change to reach the hook.
+    await new Promise((resolve) =>
+      requestAnimationFrame(() => requestAnimationFrame(resolve)),
+    );
+    await expect.element(search).toHaveValue("iron ");
+  });
+
+  it("takes the query from the URL on back navigation", async () => {
+    setSearch("?q=iron");
+    const queryClient = labQueryClient();
+    queryClient.setQueryData(
+      queryKeys.labResultsOverview("en"),
+      overviewWithAbnormalMarker,
+    );
+
+    const { screen } = await renderWithApp(<LabList />, { queryClient });
+    const search = screen.getByRole("searchbox", { name: "Search biomarkers" });
+    await expect.element(search).toHaveValue("iron");
+
+    await screen.getByRole("button", { name: "Reset" }).click();
+    await expect.poll(currentSearch).toBe("");
+    await expect.element(search).toHaveValue("");
+
+    window.history.back();
+    await expect.poll(currentSearch).toBe("?q=iron");
+    await expect.element(search).toHaveValue("iron");
+  });
+
   it("memoizes setCategoryFilter callback identity across re-renders", async () => {
     const callbacks: unknown[] = [];
     const { screen } = await renderWithApp(

src/components/labs/use-lab-list-filters.ts

@@ -19,20 +19,29 @@ export function useLabListFilters() {
       serialise: labListFiltersToSearch,
     });
 
-  // Debounced search draft
-  const [searchDraft, setSearchDraft] = useState(filters.q ?? "");
-  const [lastUrlQ, setLastUrlQ] = useState(filters.q ?? "");
+  // The field edits a draft; the URL takes it, trimmed, 200 ms after the
+  // last keystroke. A `q` the URL gains from elsewhere (back/forward, a
+  // shared link, Reset) replaces the draft. One the draft wrote itself
+  // does not: adopting it would drop a trailing space ("vitamin " became
+  // "vitamin", and the next letter ran on), or a letter typed while the
+  // write was landing.
+  const urlQ = filters.q ?? "";
+  const [searchDraft, setSearchDraft] = useState(urlQ);
+  const [lastUrlQ, setLastUrlQ] = useState(urlQ);
+  const [ownWrite, setOwnWrite] = useState<string | null>(null);
   const searchInputRef = useRef<HTMLInputElement | null>(null);
 
-  if ((filters.q ?? "") !== lastUrlQ) {
-    setLastUrlQ(filters.q ?? "");
-    setSearchDraft(filters.q ?? "");
+  if (urlQ !== lastUrlQ) {
+    setLastUrlQ(urlQ);
+    if (urlQ === ownWrite) setOwnWrite(null);
+    else if (urlQ !== searchDraft.trim()) setSearchDraft(urlQ);
   }
 
   useEffect(() => {
     const handle = setTimeout(() => {
       const trimmed = searchDraft.trim();
       if (trimmed === (filters.q ?? "")) return;
+      setOwnWrite(trimmed);
       applyFilters(
         (current) =>
           trimmed === (current.q ?? "")