fix(web): closing the last Browser tab hides the right panel again

apps/web/src/components/preview/agentOpenedPreviews.test.ts

@@ -258,7 +258,7 @@ describe("followPreviewSessions", () => {
     useRightPanelStore
       .getState()
       .byThreadKey[threadKey]?.surfaces.filter((surface) => surface.kind === "preview")
-      .map((surface) => surface.id);
+      .map((surface) => surface.id) ?? [];
 
   it("adds visible sessions as tabs and drops tabs whose session ended", () => {
     setup();

apps/web/src/hooks/useRightPanelSurfaceClose.ts

@@ -88,8 +88,10 @@ export function useRightPanelSurfaceClose(input: {
       (left, right) => Number(left.id === activeId) - Number(right.id === activeId),
     );
     for (const surface of ordered) {
-      releaseSurfaceResources(threadRef, surface);
+      // Releasing a browser drops its session, which prunes the tab automatically; close it
+      // first so the close stays a user action that hides the panel after the last tab.
       store.closeSurface(threadRef, surface.id);
+      releaseSurfaceResources(threadRef, surface);
     }
     const nextActiveSurface = selectActiveRightPanelSurface(
       useRightPanelStore.getState().byThreadKey,

apps/web/src/rightPanelStore.test.ts

@@ -1181,6 +1181,24 @@ describe("rightPanelStore", () => {
     });
   });
 
+  it("hides the panel when browser reconciliation removes its last session tab", () => {
+    const store = useRightPanelStore.getState();
+    store.openBrowser(refA, "tab-a");
+    store.reconcileBrowserSurfaces(refA, []);
+    expect(selectThreadRightPanelState(useRightPanelStore.getState().byThreadKey, refA)).toEqual({
+      isOpen: false,
+      activeSurfaceId: null,
+      surfaces: [],
+    });
+
+    store.openBrowser(refA, "tab-b");
+    store.open(refA, "diff");
+    store.reconcileBrowserSurfaces(refA, []);
+    expect(
+      selectThreadRightPanelState(useRightPanelStore.getState().byThreadKey, refA),
+    ).toMatchObject({ isOpen: true, activeSurfaceId: "diff" });
+  });
+
   it("moves a surface to a new index and keeps it there through browser reconciliation", () => {
     const store = useRightPanelStore.getState();
     store.openTerminal(refA, "term-1");

apps/web/src/rightPanelStore.ts

@@ -972,8 +972,14 @@ export const useRightPanelStore = create<RightPanelStoreState>()(
               (surface) => surface.id === current.activeSurfaceId,
             );
             const fallbackBrowser = surfaces.find((surface) => surface.kind === "preview");
+            // A closed session tab was the last surface: hide the panel rather than leave it on
+            // the empty picker, as a user close would. A dropped `browser:new` placeholder does not count.
+            const closedLastSessionTab =
+              surfaces.length === 0 &&
+              current.surfaces.some((surface) => surface.kind === "preview" && surface.resourceId);
             return {
               ...current,
+              isOpen: closedLastSessionTab ? false : current.isOpen,
               surfaces,
               activeSurfaceId: activeStillExists
                 ? current.activeSurfaceId