fix(agent-sidebar): remember a drag to the minimum width (single-writer width)

src/components/layout/app-shell/AppShell.tsx

@@ -391,8 +391,11 @@ function AppShellInner({
   // viewport. Falls back to half the viewport when no wider width was ever
   // recorded (lastOpenWidth still at the MIN_WIDTH floor).
   const reopenWidth = () => {
+    // `>=`: lastOpenWidth === MIN_WIDTH is a real remembered width (user dragged
+    // to the floor), not "no saved width" — restore it instead of falling back
+    // to half-viewport. Only the seed default (0, below the floor) falls back.
     const remembered =
-      lastOpenWidth > MIN_WIDTH
+      lastOpenWidth >= MIN_WIDTH
         ? lastOpenWidth
         : (windowWidth || 1000) * 0.5;
     return Math.min(Math.max(remembered, MIN_WIDTH), maxWidthRef.current);
@@ -433,7 +436,11 @@ function AppShellInner({
     if (sidebarWidth <= MIN_WIDTH) {
       setSidebarWidth(reopenWidth());
     } else {
-      setSidebarWidth(MIN_WIDTH);
+      // Collapse via seedWidth (NOT setSidebarWidth): now that a drag to exactly
+      // MIN_WIDTH is a remembered width, collapsing to the floor must NOT record
+      // it as the open width — otherwise expanding would have nothing else to
+      // restore. seedWidth shrinks the view without touching lastOpenWidth.
+      seedWidth(MIN_WIDTH);
     }
   };
 

src/context/sidebar/SidebarProvider.test.tsx

@@ -13,14 +13,21 @@ afterEach(() => {
 });
 
 function TestConsumer() {
-  const { sidebarWidth, setSidebarWidth, lastOpenWidth } = useSidebarWidth();
+  const { sidebarWidth, setSidebarWidth, seedWidth, lastOpenWidth } =
+    useSidebarWidth();
   return (
     <div>
       <span data-testid="width">{sidebarWidth}</span>
       <span data-testid="last-open">{lastOpenWidth}</span>
       <button onClick={() => setSidebarWidth(500)}>Resize</button>
       <button onClick={() => setSidebarWidth(0)}>Close</button>
-      <button onClick={() => setSidebarWidth(350)}>Collapse</button>
+      {/* Collapse uses seedWidth (what AppShell's collapse button does) — sets
+          the rendered width to the floor WITHOUT recording it as the open
+          width, so reopen restores the prior size. */}
+      <button onClick={() => seedWidth(350)}>Collapse</button>
+      {/* A user DRAG that lands exactly on the floor goes through
+          setSidebarWidth and IS a real chosen width (must be recorded). */}
+      <button onClick={() => setSidebarWidth(350)}>DragFloor</button>
     </div>
   );
 }
@@ -163,6 +170,35 @@ describe("SidebarProvider persistence", () => {
     expect(localStorage.getItem(SIDEBAR_WIDTH_STORAGE_KEY)).toBe("500");
   });
 
+  it("a DRAG to exactly minOpenWidth is recorded as the open width (regression: floor was dropped by a strict `>`)", () => {
+    // The bug: dragging the rail all the way to its minimum (width ===
+    // minOpenWidth) was discarded — handleSetWidth/reopenWidth used a strict
+    // `>`, so lastOpenWidth kept the previous larger size and reopen restored
+    // THAT, not the dragged floor. A drag to the floor is a real chosen width
+    // and must stick. (Contrast the collapse test above, which seeds the floor
+    // and must NOT record.)
+    render(
+      <SidebarProvider
+        defaultWidth={0}
+        persistKey={SIDEBAR_WIDTH_STORAGE_KEY}
+        minOpenWidth={350}
+        maxOpenWidth={2000}
+      >
+        <TestConsumer />
+      </SidebarProvider>,
+    );
+    act(() => {
+      fireEvent.click(screen.getByText("Resize")); // open wide (500) first
+    });
+    act(() => {
+      fireEvent.click(screen.getByText("DragFloor")); // drag down to the floor
+    });
+    expect(screen.getByTestId("width").textContent).toBe("350");
+    // The floor IS now the remembered open width — in memory AND storage.
+    expect(screen.getByTestId("last-open").textContent).toBe("350");
+    expect(localStorage.getItem(SIDEBAR_WIDTH_STORAGE_KEY)).toBe("350");
+  });
+
   it("rehydrates a valid stored width", () => {
     localStorage.setItem(SIDEBAR_WIDTH_STORAGE_KEY, "480");
     render(

src/context/sidebar/SidebarProvider.tsx

@@ -168,33 +168,21 @@ export function SidebarProvider({
       // values (e.g. from a wider viewport, or a bad client) are ignored.
       if (stored === null) return;
       if (stored < min || stored > max) return;
+      // FIRST REAL WRITER WINS. Once a genuine user gesture (a drag through
+      // handleSetWidth) has recorded a width, widthHydratedRef is already true,
+      // and the late-arriving hydration read must NOT touch anything — not the
+      // rendered width AND not the remembered reopen width (lastOpenWidth).
+      // Previously `setLastOpenWidth(stored)` ran unconditionally here, so a GET
+      // that resolved just AFTER a drag overwrote the dragged size: the user
+      // dragged to B, the stale read stamped lastOpenWidth back to A, and the
+      // next reopen came back at A instead of B. A placeholder seed (seedWidth,
+      // used by AppShell's controlled-open effect on reload) does NOT set the
+      // flag, so the stored width still wins that race exactly once and reload
+      // restore keeps working.
+      if (widthHydratedRef.current) return;
+      widthHydratedRef.current = true;
       setLastOpenWidth(stored);
-      // Reconcile the fetched width into the RENDERED state, not just the
-      // reopen memory (`lastOpenWidth`). Without this the fetched width is
-      // stranded — it only surfaces on the NEXT open gesture (reopenWidth reads
-      // lastOpenWidth), which is why close-then-reopen restored it but a reload
-      // did not.
-      //
-      // The first time we apply, the fetched stored width overrides whatever is
-      // rendered — even a non-zero PLACEHOLDER seed. On reload AppShell's
-      // controlled-open effect synchronously seeds a half-viewport / floor width
-      // the instant the host's persisted open:true hydrates (lastOpenWidth is
-      // still 0 at that point, so reopenWidth() returns a placeholder, not the
-      // real saved width). That seed lands BEFORE this async GET resolves, so a
-      // plain `cur <= 0` guard would discard the real stored width. Overriding
-      // once on first hydrate makes the persisted width win that race. A genuine
-      // user drag landing before the fetch sets widthHydratedRef in
-      // handleSetWidth, so it is treated as already-hydrated and is NOT
-      // clobbered — first REAL writer wins. After the one-time hydrate, the
-      // functional updater seeds only from the default floor (`cur <= 0`). No
-      // render gate: width intentionally paints default-then-hydrate to avoid an
-      // SSR mismatch.
-      if (!widthHydratedRef.current) {
-        widthHydratedRef.current = true;
-        setSidebarWidth(stored);
-      } else {
-        setSidebarWidth((cur) => (cur <= 0 ? stored : cur));
-      }
+      setSidebarWidth(stored);
     };
 
     const persistence = persistenceRef.current;
@@ -219,10 +207,11 @@ export function SidebarProvider({
     apply(readPersistedWidth(persistKey, min, max));
   }, []);
 
-  // Persist every meaningful open width; skip closed/collapsed-to-floor states
-  // so a close OR a collapse (width === minOpenWidth) doesn't wipe the
-  // remembered size. The injected server persistence wins; the legacy
-  // localStorage `persistKey` is written only when no persistence is wired.
+  // Persist a width. Called by handleSetWidth only for real open widths
+  // (>= minOpenWidth, including a drag to the floor); a close (0) is gated out
+  // upstream and a collapse never flows here (AppShell collapses via seedWidth).
+  // The injected persistence wins; the legacy localStorage `persistKey` is
+  // written only when no persistence is wired.
   const persist = useCallback(
     (width: number) => {
       const persistence = persistenceRef.current;
@@ -252,16 +241,16 @@ export function SidebarProvider({
   const handleSetWidth = useCallback(
     (width: number) => {
       setSidebarWidth(width);
-      // Strict `>`: a width of exactly `minOpenWidth` is the collapsed floor
-      // (AppShell collapses by setting the width to MIN_WIDTH), not a real open
-      // width — recording it would erase the user's chosen drag size and make
-      // reopen fall back to the default. Only widths strictly above the floor
-      // count as the remembered open width.
-      if (width > minOpenWidth) {
-        // A real user drag landing before the async fetch resolves is the
-        // authoritative width — mark hydrated so the late-arriving stored width
-        // does NOT overwrite it (first REAL writer wins). A placeholder seed at
-        // or below the floor never reaches here, so it can't claim hydration.
+      // `>=`, not `>`: minOpenWidth is the narrowest VALID OPEN width (the drag
+      // floor), so a user dragging all the way to it (width === minOpenWidth) is
+      // a deliberate choice and MUST be remembered. Only a CLOSE (width 0, below
+      // the floor) is excluded. The collapse button does NOT come through here —
+      // it uses `seedWidth`, so collapsing to the floor never overwrites the
+      // remembered open width. (The old strict `>` dropped a drag-to-minimum,
+      // leaving lastOpenWidth at the previous size, so reopen ignored it.)
+      if (width >= minOpenWidth) {
+        // A real user drag is the authoritative width — mark hydrated so a
+        // late-arriving stored width can't overwrite it (first REAL writer wins).
         widthHydratedRef.current = true;
         setLastOpenWidth(width);
         persist(width);

src/hooks/agent-chat/useAgentTabs.ts

@@ -297,6 +297,15 @@ export interface UseAgentTabsResult {
   /** Sidebar open/closed — persisted, shared across hosts. */
   open: boolean;
   setOpen: (open: boolean) => void;
+  /** Shared open-sidebar width (px) — the SAME persisted record as
+   *  open/tabs/activeTabId. 0 = no saved width (host default). */
+  width: number;
+  /** Persist a new open-sidebar width. Routes through the strip's single
+   *  debounced writer so a width save can never race/clobber the open or tabs
+   *  fields (and vice-versa) — there is exactly ONE writer for the record. The
+   *  host wires this to the kit's width-persistence `set` so a drag-end folds
+   *  width into the same PUT as the rest of the strip. */
+  setWidth: (width: number) => void;
   /** Open tabs as conversation ids (conversation ids + local draft ids),
    *  strip order. Map to titled ChatTabs with deriveTabTitles. */
   tabs: string[];
@@ -329,9 +338,11 @@ export interface UseAgentTabsResult {
  * mutation is pending flush so it can't clobber newer local state.
  *
  * The persisted record (AgentSidebarState) also carries the shared
- * open-sidebar `width`; the strip never MANAGES width (a host-side width
- * persistence owns the read/write — see api-client-shared), but it preserves
- * the hydrated width through every reducer so a tab PUT never drops it.
+ * open-sidebar `width`. The strip is the SINGLE writer of the whole record:
+ * `setWidth` folds a width change into the same debounced PUT as open/tabs, and
+ * every reducer preserves the hydrated width — so width and open/tabs can never
+ * race or clobber each other (the host's width-persistence `set` routes here
+ * instead of doing its own GET→merge→PUT).
  *
  * `persistence` and `resolveTabs` are read through refs — their identities
  * may change per render without retriggering fetches.
@@ -546,6 +557,15 @@ export function useAgentTabs(
     (open: boolean) => mutate((s) => (s.open === open ? s : { ...s, open })),
     [mutate],
   );
+  // Width is just another field of the one record — routing it through the same
+  // `mutate`/`schedulePut` makes the strip the SINGLE writer, so a width PUT
+  // carries the live open/tabs and an open PUT carries the live width. No second
+  // writer, no read-modify-write race (the old standalone width persister did a
+  // GET→merge→PUT that could clobber a freshly-written open).
+  const setWidth = useCallback(
+    (width: number) => mutate((s) => (s.width === width ? s : { ...s, width })),
+    [mutate],
+  );
   const selectTab = useCallback((id: string) => mutate((s) => selectTabState(s, id)), [mutate]);
   const closeTab = useCallback(
     (id: string) => mutate((s) => closeTabState(s, id, freshDraft)),
@@ -578,6 +598,8 @@ export function useAgentTabs(
   return {
     open: strip.open,
     setOpen,
+    width: strip.width,
+    setWidth,
     tabs,
     activeTabId: strip.activeTabId,
     hydrated,