fix(webui): preserve session drag contracts

webui/src/components/ChatList.tsx

@@ -221,7 +221,11 @@ export const ChatList = memo(function ChatList({
     const targetIndex = reordered.indexOf(targetKey);
     if (targetIndex < 0) return;
     reordered.splice(targetIndex + (edge === "after" ? 1 : 0), 0, draggedSessionKey);
-    onReorderSessions(reordered);
+    const groupedKeys = new Set(keys);
+    onReorderSessions([
+      ...reordered,
+      ...sessionOrder.filter((key) => !groupedKeys.has(key)),
+    ]);
   };
 
   return (

webui/src/components/thread/ThreadComposer.tsx

@@ -1730,9 +1730,8 @@ export function ThreadComposer({
 
   const previewSessionDrop = useCallback((event: React.DragEvent) => {
     if (!hasDraggedSession(event.dataTransfer)) return false;
-    event.preventDefault();
-    event.dataTransfer.dropEffect = "copy";
     if (disabled) {
+      event.dataTransfer.dropEffect = "none";
       setSessionDragPreview(null);
       return true;
     }
@@ -1744,9 +1743,12 @@ export function ThreadComposer({
       (candidate) => candidate.session_key === mention.session_key,
     );
     if (!mention || (!alreadySelected && activeSessionMentions.length >= SESSION_MENTIONS_LIMIT)) {
+      event.dataTransfer.dropEffect = "none";
       setSessionDragPreview(null);
       return true;
     }
+    event.preventDefault();
+    event.dataTransfer.dropEffect = "copy";
     const start = textareaRef.current?.selectionStart ?? value.length;
     const end = textareaRef.current?.selectionEnd ?? start;
     setSessionDragPreview((current) => (

webui/src/tests/chat-list.test.tsx

@@ -88,6 +88,8 @@ describe("ChatList", () => {
       session({ chatId: "alpha", title: "Alpha" }),
       session({ chatId: "bravo", title: "Bravo" }),
       session({ chatId: "charlie", title: "Charlie" }),
+      session({ chatId: "old-a", title: "Old A" }),
+      session({ chatId: "old-b", title: "Old B" }),
     ];
     const { rerender } = render(
       <ChatList
@@ -99,6 +101,8 @@ describe("ChatList", () => {
         onRequestRename={vi.fn()}
         onToggleArchive={vi.fn()}
         onReorderSessions={onReorderSessions}
+        archivedKeys={["websocket:old-a", "websocket:old-b"]}
+        sessionOrder={sessions.map((item) => item.key)}
       />,
     );
     const dataTransfer = {
@@ -117,6 +121,8 @@ describe("ChatList", () => {
       "websocket:bravo",
       "websocket:charlie",
       "websocket:alpha",
+      "websocket:old-a",
+      "websocket:old-b",
     ]);
 
     rerender(
@@ -129,10 +135,13 @@ describe("ChatList", () => {
         onRequestRename={vi.fn()}
         onToggleArchive={vi.fn()}
         onReorderSessions={onReorderSessions}
+        archivedKeys={["websocket:old-a", "websocket:old-b"]}
         sessionOrder={[
           "websocket:bravo",
           "websocket:charlie",
           "websocket:alpha",
+          "websocket:old-a",
+          "websocket:old-b",
         ]}
         sort="manual"
       />,

webui/src/tests/thread-composer.test.tsx

@@ -1647,6 +1647,28 @@ describe("ThreadComposer", () => {
     });
   });
 
+  it("rejects session drops that are unavailable to the composer", () => {
+    render(
+      <ThreadComposer
+        onSend={vi.fn()}
+        placeholder="Type your message..."
+        sessions={[]}
+      />,
+    );
+    const input = screen.getByLabelText("Message input");
+    const dataTransfer = {
+      types: [SESSION_DRAG_TYPE],
+      effectAllowed: "copyMove",
+      dropEffect: "copy",
+      files: [],
+      getData: () => "websocket:current",
+    };
+
+    expect(fireEvent.dragEnter(input, { dataTransfer })).toBe(true);
+    expect(fireEvent.dragOver(input, { dataTransfer })).toBe(true);
+    expect(screen.queryByTestId("composer-session-drag-preview")).not.toBeInTheDocument();
+  });
+
   it("disambiguates duplicate and capability-colliding session names", () => {
     render(
       <ThreadComposer