fix(tui): delineate assistant responses from details (#31087)

* fix(tui): delineate assistant responses from details

Add a muted Response marker before assistant text when thinking/tool details are visible so reasoning and final output do not visually run together.

* fix(tui): account for response separator height

Keep virtual transcript estimates aligned with the new response separator and avoid allocating trimmed copies of long assistant text.

* fix(tui): gate response separator estimate on details

Only add response-separator height when assistant details actually render, and use a non-allocating body-text check.

* fix(tui): skip empty detail height estimates

Do not add virtual transcript height for assistant details when no thinking or tool detail UI will render.

* fix(tui): estimate details by section visibility

Pass resolved thinking/tool visibility into virtual height estimates so hidden detail sections do not reserve response-separator rows.

ui-tui/src/components/messageLine.tsx

@@ -109,6 +109,8 @@ export const MessageLine = memo(function MessageLine({
   const showDetails =
     (toolsMode !== 'hidden' && Boolean(msg.tools?.length)) || (thinkingMode !== 'hidden' && Boolean(thinking))
 
+  const showResponseSeparator = shouldShowResponseSeparator(msg, showDetails)
+
   const content = (() => {
     if (msg.kind === 'slash') {
       return <Text color={t.color.muted}>{msg.text}</Text>
@@ -195,6 +197,17 @@ export const MessageLine = memo(function MessageLine({
         </Box>
       )}
 
+      {showResponseSeparator && (
+        <Box marginBottom={1}>
+          <NoSelect flexShrink={0} fromLeftEdge width={gutterWidth}>
+            <Text color={t.color.border}>└─ </Text>
+          </NoSelect>
+          <Text color={t.color.muted} dim>
+            Response
+          </Text>
+        </Box>
+      )}
+
       <Box>
         <NoSelect flexShrink={0} fromLeftEdge width={gutterWidth}>
           <Text bold={msg.role === 'user'} color={prefix}>
@@ -208,6 +221,9 @@ export const MessageLine = memo(function MessageLine({
   )
 })
 
+export const shouldShowResponseSeparator = (msg: Msg, showDetails: boolean): boolean =>
+  msg.role === 'assistant' && showDetails && /\S/.test(msg.text)
+
 interface MessageLineProps {
   cols: number
   compact?: boolean