fix(workspace): 统一折叠思考块代码以消除流式抖动

- 移除思考链代码块按长度判断的折叠策略，改为默认折叠\n- 为思考内容与 reasoning-only 路径添加代码块默认折叠渲染\n- 调整流式 Markdown 渲染参数，降低流式重排与闪烁

frontend/src/components/ai-elements/reasoning.tsx

@@ -168,18 +168,56 @@ export type ReasoningContentProps = ComponentProps<
 };
 
 export const ReasoningContent = memo(
-  ({ className, children, ...props }: ReasoningContentProps) => (
-    <CollapsibleContent
-      className={cn(
-        "mt-4 text-sm",
-        "data-[state=closed]:fade-out-0 data-[state=closed]:slide-out-to-top-2 data-[state=open]:slide-in-from-top-2 text-muted-foreground data-[state=closed]:animate-out data-[state=open]:animate-in outline-none",
-        className,
-      )}
-      {...props}
-    >
-      <Streamdown {...props}>{children}</Streamdown>
-    </CollapsibleContent>
-  ),
+  ({ className, children, ...props }: ReasoningContentProps) => {
+    const { isStreaming } = useReasoning();
+    const thinkingComponents = {
+      code: ({ className, children, ...codeProps }: ComponentProps<"code">) => {
+        const isBlock =
+          typeof className === "string" && className.includes("language-");
+        if (!isBlock) {
+          return (
+            <code className={className} {...codeProps}>
+              {children}
+            </code>
+          );
+        }
+        return (
+          <details className="my-2 rounded-md border">
+            <summary className="text-muted-foreground cursor-pointer px-3 py-2 text-xs select-none">
+              Show code
+            </summary>
+            <pre className="bg-muted/40 overflow-x-auto border-t p-3 text-xs">
+              <code className={className} {...codeProps}>
+                {children}
+              </code>
+            </pre>
+          </details>
+        );
+      },
+    };
+    return (
+      <CollapsibleContent
+        className={cn(
+          "mt-4 text-sm",
+          "data-[state=closed]:fade-out-0 data-[state=closed]:slide-out-to-top-2 data-[state=open]:slide-in-from-top-2 text-muted-foreground data-[state=closed]:animate-out data-[state=open]:animate-in outline-none",
+          className,
+        )}
+        {...props}
+      >
+        {isStreaming ? (
+          <div className="whitespace-pre-wrap break-words">{children}</div>
+        ) : (
+          <Streamdown
+            isAnimating={false}
+            parseIncompleteMarkdown={true}
+            components={thinkingComponents}
+          >
+            {children}
+          </Streamdown>
+        )}
+      </CollapsibleContent>
+    );
+  },
 );
 
 Reasoning.displayName = "Reasoning";

frontend/src/components/workspace/messages/markdown-content.tsx

@@ -28,6 +28,7 @@ export type MarkdownContentProps = {
 /** Renders markdown content. */
 export function MarkdownContent({
   content,
+  isLoading,
   rehypePlugins,
   className,
   remarkPlugins = streamdownPlugins.remarkPlugins,
@@ -66,6 +67,8 @@ export function MarkdownContent({
   return (
     <MessageResponse
       className={className}
+      isAnimating={isLoading}
+      parseIncompleteMarkdown={!isLoading}
       remarkPlugins={remarkPlugins}
       rehypePlugins={rehypePlugins}
       components={components}

frontend/src/components/workspace/messages/message-group.tsx

@@ -12,7 +12,7 @@ import {
   SquareTerminalIcon,
   WrenchIcon,
 } from "lucide-react";
-import { useMemo, useState } from "react";
+import { useMemo, useState, type ComponentProps } from "react";
 import type { BundledLanguage } from "shiki";
 
 import {
@@ -40,7 +40,6 @@ import { Tooltip } from "../tooltip";
 
 import { MarkdownContent } from "./markdown-content";
 
-const TOOL_CONTENT_COLLAPSE_THRESHOLD = 320;
 
 export function MessageGroup({
   className,
@@ -83,7 +82,41 @@ export function MessageGroup({
     aboveLastToolCallSteps.length + (lastToolCallStep ? 1 : 0);
   const shouldShowToolSteps =
     !!lastToolCallStep && (showAbove || aboveLastToolCallSteps.length === 0);
-  const rehypePlugins = useRehypeSplitWordsIntoSpans(isLoading);
+  // Disable per-word animation in reasoning/tool chain content to avoid
+  // repeated DOM churn while streaming updates arrive.
+  const rehypePlugins = useRehypeSplitWordsIntoSpans(false);
+  const thinkingComponents = useMemo(
+    () => ({
+      code: ({
+        className,
+        children,
+        ...props
+      }: ComponentProps<"code">) => {
+        const isBlock =
+          typeof className === "string" && className.includes("language-");
+        if (!isBlock) {
+          return (
+            <code className={className} {...props}>
+              {children}
+            </code>
+          );
+        }
+        return (
+          <details className="my-2 rounded-md border">
+            <summary className="text-muted-foreground cursor-pointer px-3 py-2 text-xs select-none">
+              {t.toolCalls.expandContent}
+            </summary>
+            <pre className="bg-muted/40 overflow-x-auto border-t p-3 text-xs">
+              <code className={className} {...props}>
+                {children}
+              </code>
+            </pre>
+          </details>
+        );
+      },
+    }),
+    [t.toolCalls.expandContent],
+  );
   return (
     <ChainOfThought
       className={cn("w-full gap-2 rounded-lg bg-white", className)}
@@ -130,6 +163,7 @@ export function MessageGroup({
                       content={step.reasoning ?? ""}
                       isLoading={isLoading}
                       rehypePlugins={rehypePlugins}
+                      components={thinkingComponents}
                     />
                   }
                 ></ChainOfThoughtStep>
@@ -185,6 +219,7 @@ export function MessageGroup({
                     content={lastReasoningStep.reasoning ?? ""}
                     isLoading={isLoading}
                     rehypePlugins={rehypePlugins}
+                    components={thinkingComponents}
                   />
                 }
               ></ChainOfThoughtStep>
@@ -227,8 +262,8 @@ function ToolCall({
     language?: BundledLanguage;
     expanded?: boolean;
   }) => {
-    const shouldCollapse = content.length > TOOL_CONTENT_COLLAPSE_THRESHOLD;
-    const shouldShowCodeBlock = !shouldCollapse || expanded;
+    // Always start collapsed in thinking blocks; user must explicitly expand.
+    const shouldShowCodeBlock = expanded;
 
     return (
       <div className="space-y-1">
@@ -417,28 +452,24 @@ function ToolCall({
       return t.toolCalls.executeCommand;
     }
     const command: string | undefined = (args as { command: string })?.command;
-    const shouldCollapse =
-      !!command && command.length > TOOL_CONTENT_COLLAPSE_THRESHOLD;
     return (
       <ChainOfThoughtStep
         key={id}
         label={description}
         icon={SquareTerminalIcon}
         action={
-          shouldCollapse ? (
-            <Button
-              className="h-7 px-3 text-xs"
-              variant="ghost"
-              onClick={(event) => {
-                event.stopPropagation();
-                setIsCommandExpanded((prev) => !prev);
-              }}
-            >
-              {isCommandExpanded
-                ? t.toolCalls.collapseContent
-                : t.toolCalls.expandContent}
-            </Button>
-          ) : undefined
+          <Button
+            className="h-7 px-3 text-xs"
+            variant="ghost"
+            onClick={(event) => {
+              event.stopPropagation();
+              setIsCommandExpanded((prev) => !prev);
+            }}
+          >
+            {isCommandExpanded
+              ? t.toolCalls.collapseContent
+              : t.toolCalls.expandContent}
+          </Button>
         }
       >
         {command && (