fix(rich-md-editor): stop the editor flashing during an agent rewrite (#5160)

* fix(rich-md-editor): stop the editor flashing during an agent rewrite

Only reveal streamed chunks that extend what's already shown. A divergent chunk
(an agent rewrite/edit, e.g. removing appended text) would collapse the document
to the partial result and flash on every chunk; now the current content is held
in place and the final result is applied once on settle. Fresh writes still
reveal live.

* fix(rich-markdown-editor): seed shown-body on settled mount and track local edits

Seed lastSyncedBodyRef from a settled (non-streaming) mount and update it on
local edits via onUpdate, so the streaming hold engages on the very first agent
rewrite chunk (no collapse/flash) and the settle still applies the rewrite that
removes a local edit.

Author: waleedlatif1

apps/sim/app/workspace/[workspaceId]/files/components/file-viewer/rich-markdown-editor/rich-markdown-editor.tsx

@@ -158,6 +158,14 @@ export function LoadedRichMarkdownEditor({
   const [initialContent] = useState<JSONContent | string>(() =>
     streamingAtMountRef.current ? '' : parseMarkdownToDoc(splitFrontmatter(content).body)
   )
+  /**
+   * The body currently shown in the editor: seeded from a settled mount, updated on local edits (via
+   * onUpdate) and on each streamed sync. The streaming tick reveals a chunk only when it extends this,
+   * so an agent rewrite holds the current content instead of collapsing to a partial result.
+   */
+  const lastSyncedBodyRef = useRef<string | null>(
+    streamingAtMountRef.current ? null : splitFrontmatter(content).body
+  )
   const onChangeRef = useRef(onChange)
   onChangeRef.current = onChange
   const onSaveShortcutRef = useRef(onSaveShortcut)
@@ -263,13 +271,12 @@ export function LoadedRichMarkdownEditor({
     },
     onUpdate: ({ editor }) => {
       const md = postProcessSerializedMarkdown(editor.getMarkdown())
+      lastSyncedBodyRef.current = md
       onChangeRef.current(applyFrontmatter(settledRef.current?.frontmatter ?? '', md))
     },
   })
   editorInstanceRef.current = editor
 
-  const lastSyncedBodyRef = useRef<string | null>(null)
-
   const wasStreamingRef = useRef(streamingAtMountRef.current)
 
   const pendingStreamBodyRef = useRef<string | null>(null)
@@ -291,6 +298,12 @@ export function LoadedRichMarkdownEditor({
           streamRafRef.current = null
           return
         }
+        const shownBody = lastSyncedBodyRef.current
+        const extendsShown = shownBody === null || pending.startsWith(shownBody)
+        if (!extendsShown) {
+          streamRafRef.current = null
+          return
+        }
         if (
           pending.length > STREAM_REPARSE_THROTTLE_THRESHOLD &&
           performance.now() - lastStreamParseAtRef.current < STREAM_REPARSE_THROTTLE_MS
@@ -303,7 +316,7 @@ export function LoadedRichMarkdownEditor({
         lastStreamParseAtRef.current = performance.now()
         const el = containerRef.current
         const pinnedToBottom = el ? el.scrollHeight - el.scrollTop - el.clientHeight < 80 : false
-        editor.setEditable(false)
+        if (editor.isEditable) editor.setEditable(false)
         editor.commands.setContent(parseMarkdownToDoc(pending), {
           contentType: 'json',
           emitUpdate: false,