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) => (