fix(webui): keep prompt rail out of narrow chat columns

webui/src/components/thread/ThreadViewport.tsx

@@ -122,6 +122,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
   const { t } = useTranslation();
   const scrollRef = useRef<HTMLDivElement>(null);
   const contentRef = useRef<HTMLDivElement>(null);
+  const messageColumnRef = useRef<HTMLDivElement>(null);
   const composerDockRef = useRef<HTMLDivElement>(null);
   const bottomRef = useRef<HTMLDivElement>(null);
   const lastConversationKeyRef = useRef<string | null>(conversationKey);
@@ -519,7 +520,11 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
               data-testid="thread-message-region"
               className="flex min-h-0 flex-1 flex-col justify-start px-3 pb-4 pt-4 sm:px-4"
             >
-              <div className="mx-auto w-full max-w-[49.5rem]">
+              <div
+                ref={messageColumnRef}
+                data-testid="thread-message-column"
+                className="mx-auto w-full max-w-[49.5rem]"
+              >
                 <ThreadMessages
                   messages={visibleMessages}
                   isStreaming={isStreaming}
@@ -567,6 +572,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
         <PromptRail
           messages={visibleMessages}
           scrollRef={scrollRef}
+          contentRef={messageColumnRef}
           bottomOffset={scrollButtonBottom}
         />
       ) : null}