perf(mobile): convert only the changed block of a streaming assistant message

apps/mobile/src/features/threads/ThreadFeed.tsx

@@ -1191,6 +1191,7 @@ const AssistantMarkdownContent = memo(function AssistantMarkdownContent(props: {
   readonly onUseArtifactTemplate?: ((template: CodexArtifactTemplate) => void) | undefined;
   readonly renderImage: MarkdownImageRenderer;
   readonly skills?: ReadonlyArray<SelectableMarkdownSkill> | undefined;
+  readonly streaming?: boolean | undefined;
 }) {
   const liveMarkdown = useLiveThreadLinkLabels(props.markdown, props.environmentId);
   const threadIcons = useThreadChipIcons(props.environmentId, liveMarkdown);
@@ -1208,6 +1209,7 @@ const AssistantMarkdownContent = memo(function AssistantMarkdownContent(props: {
       onUseArtifactTemplate={props.onUseArtifactTemplate}
       renderImage={props.renderImage}
       skills={props.skills}
+      streaming={props.streaming}
     />
   );
 });
@@ -1220,6 +1222,7 @@ function AssistantMarkdownBlocks(props: {
   readonly onUseArtifactTemplate?: ((template: CodexArtifactTemplate) => void) | undefined;
   readonly renderImage: MarkdownImageRenderer;
   readonly skills?: ReadonlyArray<SelectableMarkdownSkill> | undefined;
+  readonly streaming?: boolean | undefined;
 }) {
   const blocks = useMemo(
     () =>
@@ -1262,6 +1265,7 @@ function AssistantMarkdownBlocks(props: {
           textStyle={props.markdownStyles.nativeTextStyle}
           {...props.linkHandlers}
           renderImage={props.renderImage}
+          streaming={props.streaming}
         />
       );
     });
@@ -2266,6 +2270,7 @@ function renderFeedEntry(
               onUseArtifactTemplate={props.onUseArtifactTemplate}
               renderImage={props.renderMarkdownImage}
               skills={props.skills}
+              streaming={message.streaming}
             />
           </MarkdownImageAvailableWidthContext>
         ) : null}

apps/mobile/src/lib/markdownDocument.ts

@@ -287,25 +287,35 @@ export function markdownWithHtml(markdown: string): string {
 // Start only at the beginning of a backtick run to avoid retrying every suffix.
 const INLINE_CODE_SPAN = /(?<!`)`+([^`\r\n]+)`+/g;
 
-export function markdownWithInlineFileLinks(markdown: string, fileLink: RegExp): string {
-  return transformNativeMarkdown(markdown, fileLink, false);
+/**
+ * File links are rewritten only when some inline-code span of the document looks like a path.
+ * That is a decision about the whole document, so a caller converting it in pieces passes it on.
+ */
+export function markdownMayLinkFiles(markdown: string, fileLink: RegExp): boolean {
+  for (const [, code] of markdown.matchAll(INLINE_CODE_SPAN)) {
+    if (fileLink.test(code!.trim())) return true;
+  }
+  return false;
+}
+
+export function markdownWithInlineFileLinks(
+  markdown: string,
+  fileLink: RegExp,
+  linkFiles?: boolean,
+): string {
+  return transformNativeMarkdown(markdown, fileLink, false, linkFiles);
 }
 
 function transformNativeMarkdown(
   markdown: string,
   fileLink: RegExp | undefined,
   tableBreaks: boolean,
+  linkFilesOverride?: boolean,
 ): string {
   const breakTables = tableBreaks && mayHaveTableUnderText(markdown);
-  let linkFiles = false;
-  if (fileLink) {
-    for (const [, code] of markdown.matchAll(INLINE_CODE_SPAN)) {
-      if (fileLink.test(code!.trim())) {
-        linkFiles = true;
-        break;
-      }
-    }
-  }
+  const linkFiles = fileLink
+    ? (linkFilesOverride ?? markdownMayLinkFiles(markdown, fileLink))
+    : false;
   if (!linkFiles && !breakTables) return markdown;
   const edits: MarkdownEdit[] = [];
   const fileEdits: MarkdownEdit[] = [];
@@ -441,7 +451,16 @@ export function parseMarkdownDocument(markdown: string): Root {
 }
 
 export function markdownForNativeRendering(markdown: string, fileLink: RegExp): string {
-  return transformNativeMarkdown(markdownForNativeParsing(markdown), fileLink, true);
+  return markdownRenderParsed(markdownForNativeParsing(markdown), fileLink);
+}
+
+/** What `markdownForNativeRendering` does after `markdownForNativeParsing`. */
+export function markdownRenderParsed(
+  parsed: string,
+  fileLink: RegExp,
+  linkFiles?: boolean,
+): string {
+  return transformNativeMarkdown(parsed, fileLink, true, linkFiles);
 }
 
 /** Native media slots only cover standalone images; nested file embeds must become links. */