fix(issues): wire swimlane's outer scroller into tab scroll restoration

Review blocker on #5403: board/list/issue-detail register their scroll
containers with the tab session memento protocol, but the swimlane outer
scroller did not — under the single-router architecture an inactive tab
unmounts, so a deep-scrolled swimlane returned at top after a tab switch
or reload.

Same wiring as the other surfaces: data-tab-scroll-root="swimlane" for
capture, useRestoredScrollRef in the scroller's attach callback for the
pre-paint assignment, and the saved offset into the lane Virtuoso's
initialScrollTop. Regression test asserts both the capture marker and the
restored offset.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/views/issues/components/swimlane-view.test.tsx

@@ -3,6 +3,7 @@ import { render, screen, fireEvent, act, waitFor } from "@testing-library/react"
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 import { SwimLaneView } from "./swimlane-view";
 import { IssueContextMenuProvider } from "../actions";
+import { ScrollRestorationProvider } from "../../platform";
 import type { Issue } from "@multica/core/types";
 import { I18nProvider } from "@multica/core/i18n/react";
 import enCommon from "../../locales/en/common.json";
@@ -1809,3 +1810,36 @@ describe("SwimLaneView", () => {
     expect(screen.queryByText("Batch Sub-issue")).toBeNull();
   });
 });
+
+describe("SwimLaneView tab-session scroll restoration (MUL-4741)", () => {
+  it("registers the outer scroller for memento capture and restores the saved offset at attach", () => {
+    const adapter = {
+      get: (key: string) =>
+        key === "swimlane" ? { top: 240, height: 2000 } : undefined,
+    };
+    const qc = new QueryClient({
+      defaultOptions: { queries: { retry: false, gcTime: 0 } },
+    });
+    const { container } = render(
+      <QueryClientProvider client={qc}>
+        <I18nProvider resources={TEST_RESOURCES} locale="en">
+          <IssueContextMenuProvider>
+            <ScrollRestorationProvider adapter={adapter}>
+              <SwimLaneView issues={mockIssues} onMoveIssue={vi.fn()} />
+            </ScrollRestorationProvider>
+          </IssueContextMenuProvider>
+        </I18nProvider>
+      </QueryClientProvider>,
+    );
+
+    const scroller = container.querySelector<HTMLElement>(
+      '[data-tab-scroll-root="swimlane"]',
+    );
+    // Capture side: the coordinator scans [data-tab-scroll-root] — without
+    // the marker, leaving the tab never saves the swimlane offset.
+    expect(scroller).not.toBeNull();
+    // Restore side: the ref-attach assignment applies the saved offset
+    // before first paint (jsdom has no layout, so no clamping applies).
+    expect(scroller!.scrollTop).toBe(240);
+  });
+});

packages/views/issues/components/swimlane-view.tsx

@@ -55,12 +55,8 @@ import { ProjectIcon } from "../../projects/components/project-icon";
 import { ActorAvatar } from "../../common/actor-avatar";
 import { VirtuosoSeed } from "../../common/virtuoso-seed";
 
-// A swimlane row (header + one row of card cells) is ~300px+ tall — a
-// viewport fits ~3. The generic VIRTUOSO_SEED_COUNT (30, sized for 36px list
-// rows) made every surface remount synchronously mount up to 30 full lanes
-// (each lane = statuses x cells x cards); 6 covers the viewport with margin.
-const SWIMLANE_LANE_SEED_COUNT = 6;
 import { DeferredPopup } from "../../common/deferred-popup";
+import { useRestoredScrollOffset, useRestoredScrollRef } from "../../platform";
 import { DeferredTooltip } from "../../common/deferred-tooltip";
 import type { ChildProgress } from "./list-row";
 import { useT } from "../../i18n";
@@ -70,6 +66,12 @@ import type { IssueCreateDefaults } from "../surface/types";
 const COLUMN_WIDTH = 280;
 const COLUMN_GAP = 16;
 
+// A swimlane row (header + one row of card cells) is ~300px+ tall — a
+// viewport fits ~3. The generic VIRTUOSO_SEED_COUNT (30, sized for 36px list
+// rows) made every surface remount synchronously mount up to 30 full lanes
+// (each lane = statuses x cells x cards); 6 covers the viewport with margin.
+const SWIMLANE_LANE_SEED_COUNT = 6;
+
 // Hoisted out of SwimLaneView so its reference is stable across renders —
 // useQueries' combine option uses it through replaceEqualDeep, but keeping
 // the function stable saves the per-render reference check.
@@ -822,6 +824,18 @@ function SwimLaneViewImpl({
   const [activeIssue, setActiveIssue] = useState<Issue | null>(null);
   // The outer scroll box is the customScrollParent for the lane Virtuoso.
   const [scrollEl, setScrollEl] = useState<HTMLDivElement | null>(null);
+  // Pull-based scroll restoration (MUL-4741): same wiring as board/list/
+  // issue-detail — ref-attach assigns the saved offset pre-paint, and the
+  // lane Virtuoso is born at it via initialScrollTop.
+  const restoredScrollTop = useRestoredScrollOffset("swimlane");
+  const restoreScrollRef = useRestoredScrollRef("swimlane");
+  const attachScroller = useCallback(
+    (el: HTMLDivElement | null) => {
+      setScrollEl(el);
+      restoreScrollRef(el);
+    },
+    [restoreScrollRef],
+  );
   const isDraggingRef = useRef(false);
   // Settle lock: held from drop until the move mutation settles, so a cache
   // change that lands mid-flight (e.g. a membership refetch) does not rebuild
@@ -1203,7 +1217,7 @@ function SwimLaneViewImpl({
       onDragOver={handleDragOver}
       onDragEnd={handleDragEnd}
     >
-      <div ref={setScrollEl} className="flex flex-1 min-h-0 gap-4 overflow-auto p-4">
+      <div ref={attachScroller} data-tab-scroll-root="swimlane" className="flex flex-1 min-h-0 gap-4 overflow-auto p-4">
         <div className="flex shrink-0 flex-col" style={{ width: `${trackWidth}px` }}>
         {/* Sticky status header row — visually matches the top of a BoardColumn */}
         <div className="sticky top-0 z-10 mb-2 bg-background/95 pb-2 backdrop-blur supports-[backdrop-filter]:bg-background/75">
@@ -1283,6 +1297,7 @@ function SwimLaneViewImpl({
               customScrollParent={scrollEl}
               data={orderedLanes}
               computeItemKey={computeLaneKey}
+              initialScrollTop={restoredScrollTop}
               initialItemCount={Math.min(orderedLanes.length, SWIMLANE_LANE_SEED_COUNT)}
               increaseViewportBy={{ top: 600, bottom: 600 }}
               components={laneComponents}