diff --git a/src/components/labs/__tests__/lab-list-url-sync.browser.test.tsx b/src/components/labs/__tests__/lab-list-url-sync.browser.test.tsx
index 0caa69e3d..028a4648a 100644
--- a/src/components/labs/__tests__/lab-list-url-sync.browser.test.tsx
+++ b/src/components/labs/__tests__/lab-list-url-sync.browser.test.tsx
@@ -10,6 +10,7 @@ import {
   setSearch,
 } from "../../../../tests/browser/url";
 import { LabList } from "../lab-list";
+import { readStoredOpen } from "../lab-list-groups";
 import type { LabOverviewResponse, LabResultDto } from "../types";
 import { useLabListFilters } from "../use-lab-list-filters";
 
@@ -212,11 +213,11 @@ describe("LabList stale category and group header handling", () => {
     await expect.element(groupButton.getByText("1")).toBeVisible();
   });
 
-  it("keeps flagged-mode default-open behavior consistent with visible filtered rows", async () => {
-    // overviewWithAbnormalMarker has 2 markers in Iron Panel:
-    // Iron (below) and Ferritin (in-range). Category server status is "below".
-    // When filtered to ?status=in-range, Iron is excluded and only Ferritin (in-range) remains.
-    // In flagged mode, the group should NOT be open by default because visible rows are all in-range.
+  it("keeps a flagged group open while a filter leaves only in-range rows", async () => {
+    // Iron (below) and Ferritin (in-range) share the Iron Panel, whose server
+    // status is "below". Filtering to in-range leaves Ferritin alone: the
+    // header now reads "All in range", but the group must stay open, since
+    // the counter says there is a match.
     setSearch("?status=in-range");
     const queryClient = labQueryClient();
     queryClient.setQueryData(queryKeys.moduleListLayout(), {
@@ -239,11 +240,53 @@ describe("LabList stale category and group header handling", () => {
 
     const { screen } = await renderWithApp(<LabList />, { queryClient });
 
-    // Group button reflects in-range status ("All in range") and is collapsed (aria-expanded="false")
     const groupButton = screen.getByRole("button", { name: /Iron Panel/i });
-    await expect.element(groupButton).toBeVisible();
     await expect.element(groupButton.getByText("All in range")).toBeVisible();
+    await expect.element(groupButton).toHaveAttribute("aria-expanded", "true");
+    await expect.element(screen.getByText("Ferritin")).toBeVisible();
+  });
+
+  it("opens every matching group while filtering and restores the closed state after reset", async () => {
+    setSearch("?q=ferr");
+    window.localStorage.clear();
+    const queryClient = labQueryClient();
+    queryClient.setQueryData(queryKeys.moduleListLayout(), {
+      version: 1,
+      lists: {
+        labs: {
+          view: "list",
+          order: [],
+          sortDir: "recentDesc",
+          groupBy: "category",
+          groupsOpen: "collapsed",
+          categoryOrder: [],
+        },
+      },
+    });
+    queryClient.setQueryData(
+      queryKeys.labResultsOverview("en"),
+      overviewWithAbnormalMarker,
+    );
+
+    const { screen } = await renderWithApp(<LabList />, { queryClient });
+
+    const groupButton = screen.getByRole("button", { name: /Iron Panel/i });
+    await expect.element(groupButton).toHaveAttribute("aria-expanded", "true");
+    await expect.element(screen.getByText("Ferritin")).toBeVisible();
+
+    // A toggle made while filtering holds while the filter changes...
+    await groupButton.click();
     await expect.element(groupButton).toHaveAttribute("aria-expanded", "false");
+    await screen.getByRole("combobox", { name: "Status" }).click();
+    await screen.getByRole("option", { name: "In range" }).click();
+    await expect.poll(currentSearch).toBe("?q=ferr&status=in-range");
+    await expect.element(groupButton).toHaveAttribute("aria-expanded", "false");
+
+    // ...and is dropped with the filter: "collapsed" applies again.
+    await screen.getByRole("button", { name: "Reset" }).click();
+    await expect.poll(currentSearch).toBe("");
+    await expect.element(groupButton).toHaveAttribute("aria-expanded", "false");
+    expect(readStoredOpen("collapsed")).toEqual({});
   });
 
   it("memoizes setCategoryFilter callback identity across re-renders", async () => {
diff --git a/src/components/labs/lab-list-groups.tsx b/src/components/labs/lab-list-groups.tsx
index ca6e93121..79d41d350 100644
--- a/src/components/labs/lab-list-groups.tsx
+++ b/src/components/labs/lab-list-groups.tsx
@@ -100,14 +100,14 @@ export function writeStoredOpen(
 }
 
 function openByDefault(
+  category: LabOverviewCategoryDto,
   mode: ModuleGroupsOpen,
-  status: LabOverviewCategoryDto["status"],
 ): boolean {
   if (mode === "expanded") return true;
   if (mode === "collapsed") return false;
-  // Consumers supply category.status for unfiltered categories, or the
-  // summarised status for filtered groups.
-  return status === "above" || status === "below";
+  // The server's status for the whole category, never a filtered summary:
+  // what a filter hides must not open or close a group.
+  return category.status === "above" || category.status === "below";
 }
 
 /**
@@ -115,16 +115,19 @@ function openByDefault(
  * category the person opened or closed by hand stays so in this browser
  * (keyed by the opaque category key, never by its label) until the choice
  * itself changes. Storage is read only after hydration and may be missing.
+ *
+ * While a filter is active every category that still has a match is open,
+ * whatever the choice or the stored toggles say, so a match is never hidden
+ * behind a closed header. A category opened or closed by hand during
+ * filtering stays so while the filter changes, is not written to storage,
+ * and the stored state returns once the filter is cleared.
  */
-export function useGroupsOpen(mode: ModuleGroupsOpen): {
-  isOpen: (
-    category: LabOverviewCategoryDto,
-    status: LabOverviewCategoryDto["status"],
-  ) => boolean;
-  toggle: (
-    category: LabOverviewCategoryDto,
-    status: LabOverviewCategoryDto["status"],
-  ) => void;
+export function useGroupsOpen(
+  mode: ModuleGroupsOpen,
+  filtering = false,
+): {
+  isOpen: (category: LabOverviewCategoryDto) => boolean;
+  toggle: (category: LabOverviewCategoryDto) => void;
 } {
   const mounted = useMounted();
   const stored = useMemo(
@@ -134,23 +137,31 @@ export function useGroupsOpen(mode: ModuleGroupsOpen): {
   const [toggled, setToggled] = useState<StoredOpen>({ mode, open: {} });
   const own = toggled.mode === mode ? toggled.open : {};
 
-  const isOpen = (
-    category: LabOverviewCategoryDto,
-    status: LabOverviewCategoryDto["status"],
-  ) =>
-    own[category.key] ??
-    stored[category.key] ??
-    openByDefault(mode, status);
+  const [filtered, setFiltered] = useState<{
+    filtering: boolean;
+    open: Record<string, boolean>;
+  }>({ filtering, open: {} });
+  if (filtered.filtering !== filtering) {
+    setFiltered({ filtering, open: {} });
+  }
+  const whileFiltering = filtered.filtering === filtering ? filtered.open : {};
 
-  const toggle = (
-    category: LabOverviewCategoryDto,
-    status: LabOverviewCategoryDto["status"],
-  ) => {
-    const next = {
-      ...stored,
-      ...own,
-      [category.key]: !isOpen(category, status),
-    };
+  const isOpen = (category: LabOverviewCategoryDto) =>
+    filtering
+      ? (whileFiltering[category.key] ?? true)
+      : (own[category.key] ??
+        stored[category.key] ??
+        openByDefault(category, mode));
+
+  const toggle = (category: LabOverviewCategoryDto) => {
+    if (filtering) {
+      setFiltered({
+        filtering,
+        open: { ...whileFiltering, [category.key]: !isOpen(category) },
+      });
+      return;
+    }
+    const next = { ...stored, ...own, [category.key]: !isOpen(category) };
     setToggled({ mode, open: next });
     writeStoredOpen(mode, next);
   };
diff --git a/src/components/labs/lab-list.tsx b/src/components/labs/lab-list.tsx
index 67680ca2f..1de1a0ecf 100644
--- a/src/components/labs/lab-list.tsx
+++ b/src/components/labs/lab-list.tsx
@@ -87,12 +87,11 @@ function LabTable({
 function useGroupToggle(
   category: LabOverviewCategoryDto,
   groupsOpen: GroupsOpen,
-  status: LabOverviewCategoryDto["status"],
 ) {
   const panelId = `${useId()}-panel`;
   return {
-    open: groupsOpen.isOpen(category, status),
-    onToggle: () => groupsOpen.toggle(category, status),
+    open: groupsOpen.isOpen(category),
+    onToggle: () => groupsOpen.toggle(category),
     panelId,
   };
 }
@@ -118,7 +117,6 @@ function TableGroup({
   const { open, onToggle, panelId } = useGroupToggle(
     group.category,
     groupsOpen,
-    status,
   );
   return (
     <div className={cn(LIST_SUBGRID_CLASS, "divide-y-0")}>
@@ -186,7 +184,6 @@ function CardGroup({
   const { open, onToggle, panelId } = useGroupToggle(
     group.category,
     groupsOpen,
-    status,
   );
   return (
     <section className="space-y-4">
@@ -379,7 +376,7 @@ export function LabList({
     effectiveFilters.toDay,
   );
 
-  const groupsOpen = useGroupsOpen(prefs.groupsOpen);
+  const groupsOpen = useGroupsOpen(prefs.groupsOpen, effectiveIsFiltered);
 
   // Scrub stale category filter when categories load.
   // afterPaint protects mount writes from router history initialization.