From f1ee30daa385c5f7e0af8a5cc996f68918988959 Mon Sep 17 00:00:00 2001
From: Felipe Coury <felipe.coury@gmail.com>
Date: Tue, 17 Feb 2026 16:54:22 -0300
Subject: [PATCH] tui: use source-mapped boundaries for stream resize remap

---
 codex-rs/tui/src/markdown.rs             | 208 ++++++++++++++--
 codex-rs/tui/src/markdown_render.rs      |  66 ++++-
 codex-rs/tui/src/streaming/controller.rs | 301 ++++++++++++-----------
 3 files changed, 400 insertions(+), 175 deletions(-)

diff --git a/codex-rs/tui/src/markdown.rs b/codex-rs/tui/src/markdown.rs
index f0c3ecba0f..3f23c59b56 100644
--- a/codex-rs/tui/src/markdown.rs
+++ b/codex-rs/tui/src/markdown.rs
@@ -20,10 +20,16 @@
 //! info string is `md` or `markdown` AND whose body contains a
 //! header+delimiter pair, and degrades gracefully on unclosed fences.
 use ratatui::text::Line;
+use std::ops::Range;
 use std::path::Path;
 
 use crate::table_detect;
 
+pub(crate) struct AgentRenderedWithSourceMap {
+    pub(crate) lines: Vec<Line<'static>>,
+    pub(crate) line_end_source_bytes: Vec<usize>,
+}
+
 /// Render markdown into `lines` while resolving local file-link display relative to `cwd`.
 ///
 /// Callers that already know the session working directory should pass it here so streamed and
@@ -64,8 +70,39 @@ pub(crate) fn append_markdown_agent_with_cwd(
     cwd: Option<&Path>,
     lines: &mut Vec<Line<'static>>,
 ) {
-    let normalized = unwrap_markdown_fences(markdown_source);
-    append_markdown(&normalized, width, cwd, lines);
+    let rendered = render_markdown_agent_with_source_map_with_cwd(markdown_source, width, cwd);
+    lines.extend(rendered.lines);
+}
+
+#[cfg(test)]
+pub(crate) fn render_markdown_agent_with_source_map(
+    markdown_source: &str,
+    width: Option<usize>,
+) -> AgentRenderedWithSourceMap {
+    render_markdown_agent_with_source_map_with_cwd(markdown_source, width, /*cwd*/ None)
+}
+
+pub(crate) fn render_markdown_agent_with_source_map_with_cwd(
+    markdown_source: &str,
+    width: Option<usize>,
+    cwd: Option<&Path>,
+) -> AgentRenderedWithSourceMap {
+    let normalized = unwrap_markdown_fences_with_mapping(markdown_source);
+    let rendered = crate::markdown_render::render_markdown_text_with_width_and_cwd_and_source_map(
+        &normalized.text,
+        width,
+        cwd,
+    );
+    let lines = rendered.text.lines;
+    let line_end_source_bytes = rendered
+        .line_end_input_bytes
+        .into_iter()
+        .map(|offset| normalized.raw_source_offset(offset))
+        .collect();
+    AgentRenderedWithSourceMap {
+        lines,
+        line_end_source_bytes,
+    }
 }
 
 /// Strip `` ```md ``/`` ```markdown `` fences that contain tables, emitting their content as bare
@@ -79,7 +116,64 @@ pub(crate) fn append_markdown_agent_with_cwd(
 /// The fence unwrapping is intentionally conservative: it buffers the entire fence body before
 /// deciding, and an unclosed fence at end-of-input is re-emitted with its opening line so partial
 /// streams degrade to code display.
+#[cfg(test)]
 fn unwrap_markdown_fences(markdown_source: &str) -> String {
+    unwrap_markdown_fences_with_mapping(markdown_source).text
+}
+
+#[derive(Clone, Debug)]
+struct NormalizedToRawSegment {
+    normalized_start: usize,
+    raw_start: usize,
+}
+
+struct NormalizedMarkdown {
+    text: String,
+    raw_source_len: usize,
+    segments: Vec<NormalizedToRawSegment>,
+}
+
+impl NormalizedMarkdown {
+    fn new(raw_source_len: usize, capacity: usize) -> Self {
+        Self {
+            text: String::with_capacity(capacity),
+            raw_source_len,
+            segments: Vec::new(),
+        }
+    }
+
+    fn push_source_range(&mut self, source: &str, range: Range<usize>) {
+        if range.is_empty() {
+            return;
+        }
+        let out_start = self.text.len();
+        let out_text = &source[range.clone()];
+        self.text.push_str(out_text);
+        self.segments.push(NormalizedToRawSegment {
+            normalized_start: out_start,
+            raw_start: range.start,
+        });
+    }
+
+    fn raw_source_offset(&self, normalized_offset: usize) -> usize {
+        if normalized_offset == 0 {
+            return 0;
+        }
+        if normalized_offset >= self.text.len() {
+            return self.raw_source_len;
+        }
+        let idx = self
+            .segments
+            .partition_point(|segment| segment.normalized_start < normalized_offset);
+        if idx == 0 {
+            return 0;
+        }
+        let segment = &self.segments[idx - 1];
+        segment.raw_start + normalized_offset.saturating_sub(segment.normalized_start)
+    }
+}
+
+fn unwrap_markdown_fences_with_mapping(markdown_source: &str) -> NormalizedMarkdown {
     #[derive(Clone, Copy)]
     struct Fence {
         marker: char,
@@ -159,46 +253,66 @@ fn unwrap_markdown_fences(markdown_source: &str) -> String {
         false
     }
 
+    fn content_from_ranges(source: &str, ranges: &[Range<usize>]) -> String {
+        let mut content = String::new();
+        for range in ranges {
+            content.push_str(&source[range.clone()]);
+        }
+        content
+    }
+
     enum ActiveFence {
         Passthrough(Fence),
         MarkdownCandidate {
             fence: Fence,
-            opening_line: String,
-            content: String,
+            opening_range: Range<usize>,
+            content_ranges: Vec<Range<usize>>,
         },
     }
 
-    let mut out = String::with_capacity(markdown_source.len());
+    let mut out = NormalizedMarkdown::new(markdown_source.len(), markdown_source.len());
     let mut active_fence: Option<ActiveFence> = None;
+    let mut source_offset = 0usize;
 
     for line in markdown_source.split_inclusive('\n') {
+        let line_start = source_offset;
+        source_offset += line.len();
+        let line_range = line_start..source_offset;
+
         if let Some(active) = active_fence.take() {
             match active {
                 ActiveFence::Passthrough(fence) => {
-                    out.push_str(line);
+                    out.push_source_range(markdown_source, line_range);
                     if !is_close_fence(line, fence) {
                         active_fence = Some(ActiveFence::Passthrough(fence));
                     }
                 }
                 ActiveFence::MarkdownCandidate {
                     fence,
-                    opening_line,
-                    mut content,
+                    opening_range,
+                    mut content_ranges,
                 } => {
                     if is_close_fence(line, fence) {
-                        if markdown_fence_contains_table(&content) {
-                            out.push_str(&content);
+                        if markdown_fence_contains_table(&content_from_ranges(
+                            markdown_source,
+                            &content_ranges,
+                        )) {
+                            for range in content_ranges {
+                                out.push_source_range(markdown_source, range);
+                            }
                         } else {
-                            out.push_str(&opening_line);
-                            out.push_str(&content);
-                            out.push_str(line);
+                            out.push_source_range(markdown_source, opening_range);
+                            for range in content_ranges {
+                                out.push_source_range(markdown_source, range);
+                            }
+                            out.push_source_range(markdown_source, line_range);
                         }
                     } else {
-                        content.push_str(line);
+                        content_ranges.push(line_range);
                         active_fence = Some(ActiveFence::MarkdownCandidate {
                             fence,
-                            opening_line,
-                            content,
+                            opening_range,
+                            content_ranges,
                         });
                     }
                 }
@@ -210,29 +324,31 @@ fn unwrap_markdown_fences(markdown_source: &str) -> String {
             if fence.is_markdown {
                 active_fence = Some(ActiveFence::MarkdownCandidate {
                     fence,
-                    opening_line: line.to_string(),
-                    content: String::new(),
+                    opening_range: line_range,
+                    content_ranges: Vec::new(),
                 });
             } else {
-                out.push_str(line);
+                out.push_source_range(markdown_source, line_range);
                 active_fence = Some(ActiveFence::Passthrough(fence));
             }
             continue;
         }
 
-        out.push_str(line);
+        out.push_source_range(markdown_source, line_range);
     }
 
     if let Some(active) = active_fence {
         match active {
             ActiveFence::Passthrough(_) => {}
             ActiveFence::MarkdownCandidate {
-                opening_line,
-                content,
+                opening_range,
+                content_ranges,
                 ..
             } => {
-                out.push_str(&opening_line);
-                out.push_str(&content);
+                out.push_source_range(markdown_source, opening_range);
+                for range in content_ranges {
+                    out.push_source_range(markdown_source, range);
+                }
             }
         }
     }
@@ -421,4 +537,48 @@ mod tests {
         let normalized = unwrap_markdown_fences(src);
         assert_eq!(normalized, src);
     }
+
+    #[test]
+    fn render_markdown_agent_with_source_map_offsets_align_with_line_count() {
+        let src = "alpha\n\n```md\n| A | B |\n|---|---|\n| 1 | 2 |\n```\nomega\n";
+        let rendered = render_markdown_agent_with_source_map(src, Some(80));
+        assert_eq!(rendered.lines.len(), rendered.line_end_source_bytes.len());
+
+        let mut prev = 0usize;
+        for offset in rendered.line_end_source_bytes {
+            assert!(
+                offset <= src.len(),
+                "line-end offset must stay within source bounds: {offset} > {}",
+                src.len()
+            );
+            assert!(
+                offset >= prev,
+                "line-end offsets must be non-decreasing: {offset} < {prev}"
+            );
+            prev = offset;
+        }
+    }
+
+    #[test]
+    fn unwrap_markdown_fences_mapping_preserves_raw_offsets_across_removed_fence_lines() {
+        let src = "before\n```md\n| A | B |\n|---|---|\n| 1 | 2 |\n```\nafter\n";
+        let normalized = unwrap_markdown_fences_with_mapping(src);
+        assert!(
+            !normalized.text.contains("```"),
+            "markdown table fence should be removed in normalized text"
+        );
+        let after_in_normalized = normalized
+            .text
+            .find("after")
+            .expect("expected 'after' in normalized text");
+        let after_in_raw = src.find("after").expect("expected 'after' in raw source");
+        assert_eq!(
+            normalized.raw_source_offset(after_in_normalized + 1),
+            after_in_raw + 1
+        );
+        assert_eq!(
+            normalized.raw_source_offset(normalized.text.len()),
+            src.len()
+        );
+    }
 }
diff --git a/codex-rs/tui/src/markdown_render.rs b/codex-rs/tui/src/markdown_render.rs
index ac4c6eb08f..e36f14aea0 100644
--- a/codex-rs/tui/src/markdown_render.rs
+++ b/codex-rs/tui/src/markdown_render.rs
@@ -59,6 +59,7 @@ use ratatui::text::Line;
 use ratatui::text::Span;
 use ratatui::text::Text;
 use regex_lite::Regex;
+use std::ops::Range;
 use std::path::Path;
 use std::path::PathBuf;
 use std::sync::LazyLock;
@@ -251,6 +252,11 @@ pub fn render_markdown_text(input: &str) -> Text<'static> {
     render_markdown_text_with_width(input, /*width*/ None)
 }
 
+pub(crate) struct RenderedMarkdownWithSourceMap {
+    pub(crate) text: Text<'static>,
+    pub(crate) line_end_input_bytes: Vec<usize>,
+}
+
 /// Render markdown constrained to a known terminal width.
 ///
 /// The renderer preserves table structure when possible and falls back to
@@ -273,13 +279,33 @@ pub(crate) fn render_markdown_text_with_width_and_cwd(
     width: Option<usize>,
     cwd: Option<&Path>,
 ) -> Text<'static> {
+    render_markdown_text_with_width_and_cwd_and_source_map(input, width, cwd).text
+}
+
+#[cfg(test)]
+pub(crate) fn render_markdown_text_with_width_and_source_map(
+    input: &str,
+    width: Option<usize>,
+) -> RenderedMarkdownWithSourceMap {
+    let cwd = std::env::current_dir().ok();
+    render_markdown_text_with_width_and_cwd_and_source_map(input, width, cwd.as_deref())
+}
+
+pub(crate) fn render_markdown_text_with_width_and_cwd_and_source_map(
+    input: &str,
+    width: Option<usize>,
+    cwd: Option<&Path>,
+) -> RenderedMarkdownWithSourceMap {
     let mut options = Options::empty();
     options.insert(Options::ENABLE_STRIKETHROUGH);
     options.insert(Options::ENABLE_TABLES);
-    let parser = Parser::new_ext(input, options);
-    let mut w = Writer::new(parser, width, cwd);
+    let parser = Parser::new_ext(input, options).into_offset_iter();
+    let mut w = Writer::new(parser, width, input.len(), cwd);
     w.run();
-    w.text
+    RenderedMarkdownWithSourceMap {
+        text: w.text,
+        line_end_input_bytes: w.line_end_input_bytes,
+    }
 }
 
 #[derive(Clone, Debug)]
@@ -320,10 +346,13 @@ static HASH_LOCATION_SUFFIX_RE: LazyLock<Regex> =
 /// allocation; when `None`, lines keep their intrinsic width.
 struct Writer<'a, I>
 where
-    I: Iterator<Item = Event<'a>>,
+    I: Iterator<Item = (Event<'a>, Range<usize>)>,
 {
     iter: I,
+    source_len: usize,
     text: Text<'static>,
+    line_end_input_bytes: Vec<usize>,
+    current_event_end: usize,
     styles: MarkdownStyles,
     inline_styles: Vec<Style>,
     indent_stack: Vec<IndentContext>,
@@ -349,12 +378,20 @@ where
 
 impl<'a, I> Writer<'a, I>
 where
-    I: Iterator<Item = Event<'a>>,
+    I: Iterator<Item = (Event<'a>, Range<usize>)>,
 {
-    fn new(iter: I, wrap_width: Option<usize>, cwd: Option<&Path>) -> Self {
+    fn new(
+        iter: I,
+        wrap_width: Option<usize>,
+        source_len: usize,
+        cwd: Option<&Path>,
+    ) -> Self {
         Self {
             iter,
+            source_len,
             text: Text::default(),
+            line_end_input_bytes: Vec::new(),
+            current_event_end: 0,
             styles: MarkdownStyles::default(),
             inline_styles: Vec::new(),
             indent_stack: Vec::new(),
@@ -380,9 +417,11 @@ where
     }
 
     fn run(&mut self) {
-        while let Some(ev) = self.iter.next() {
+        while let Some((ev, range)) = self.iter.next() {
+            self.current_event_end = range.end;
             self.handle_event(ev);
         }
+        self.current_event_end = self.source_len;
         self.flush_current_line();
     }
 
@@ -1569,13 +1608,13 @@ where
                     .subsequent_indent(self.current_subsequent_indent.clone().into());
                 for wrapped in adaptive_wrap_line(&line, opts) {
                     let owned = line_to_static(&wrapped).style(style);
-                    self.text.lines.push(owned);
+                    self.push_output_line(owned);
                 }
             } else {
                 let mut spans = self.current_initial_indent.clone();
                 let mut line = line;
                 spans.append(&mut line.spans);
-                self.text.lines.push(Line::from_iter(spans).style(style));
+                self.push_output_line(Line::from_iter(spans).style(style));
             }
             self.current_initial_indent.clear();
             self.current_subsequent_indent.clear();
@@ -1605,7 +1644,7 @@ where
 
         let mut spans = self.prefix_spans(pending_marker_line);
         spans.extend(line.spans);
-        self.text.lines.push(Line::from(spans).style(style));
+        self.push_output_line(Line::from(spans).style(style));
     }
 
     fn push_line(&mut self, line: Line<'static>) {
@@ -1642,13 +1681,18 @@ where
     fn push_blank_line(&mut self) {
         self.flush_current_line();
         if self.indent_stack.iter().all(|ctx| ctx.is_list) {
-            self.text.lines.push(Line::default());
+            self.push_output_line(Line::default());
         } else {
             self.push_line(Line::default());
             self.flush_current_line();
         }
     }
 
+    fn push_output_line(&mut self, line: Line<'static>) {
+        self.text.lines.push(line);
+        self.line_end_input_bytes.push(self.current_event_end);
+    }
+
     fn prefix_spans(&self, pending_marker_line: bool) -> Vec<Span<'static>> {
         let mut prefix: Vec<Span<'static>> = Vec::new();
         let last_marker_index = if pending_marker_line {
diff --git a/codex-rs/tui/src/streaming/controller.rs b/codex-rs/tui/src/streaming/controller.rs
index 691c56fd6c..f96f45cb33 100644
--- a/codex-rs/tui/src/streaming/controller.rs
+++ b/codex-rs/tui/src/streaming/controller.rs
@@ -21,14 +21,15 @@
 //! ## Resize handling
 //!
 //! On terminal width change, `StreamCore::set_width` remaps the
-//! already-emitted line count to the new width using
-//! `source_bytes_for_rendered_count`, then rebuilds the stable queue.  The
+//! already-emitted stable boundary to the new width from source-byte anchors
+//! and line-end source maps, then rebuilds the stable queue.  The
 //! caller (`App`) schedules a debounced full-transcript reflow so committed
 //! `AgentMarkdownCell`s re-render from source at the new width.
 //!
 //! ## Invariants
 //!
 //! - `emitted_stable_len <= enqueued_stable_len <= rendered_lines.len()`.
+//! - `rendered_line_end_source_bytes.len() == rendered_lines.len()`.
 //! - `raw_source` is append-only until `reset()`; never modified mid-stream.
 //! - Tail starts exactly at `enqueued_stable_len`.
 //! - During confirmed table streaming, only lines from the table header onward
@@ -37,6 +38,7 @@
 use crate::history_cell::HistoryCell;
 use crate::history_cell::{self};
 use crate::markdown::append_markdown_agent_with_cwd;
+use crate::markdown::render_markdown_agent_with_source_map_with_cwd;
 use crate::render::line_utils::prefix_lines;
 use crate::style::proposed_plan_style;
 use ratatui::prelude::Stylize;
@@ -73,12 +75,20 @@ struct StreamCore {
     raw_source: String,
     /// Full re-render of `raw_source` at `width`. Rebuilt on every committed delta.
     rendered_lines: Vec<Line<'static>>,
+    /// Per-rendered-line source byte offsets (raw source coordinates).
+    rendered_line_end_source_bytes: Vec<usize>,
     /// Lines enqueued into the commit-animation queue.
     enqueued_stable_len: usize,
     /// Lines actually emitted to scrollback.
     emitted_stable_len: usize,
     /// Session cwd used to keep local file-link display stable during stream re-renders.
     cwd: PathBuf,
+    /// Raw source boundary reached by emitted stable lines.
+    emitted_stable_source_bytes: usize,
+    /// How many emitted stable lines end exactly at `emitted_stable_source_bytes`.
+    emitted_stable_lines_at_source_byte: usize,
+    /// Total rendered lines ending at `emitted_stable_source_bytes` in current render.
+    emitted_stable_lines_at_source_byte_total: usize,
 }
 
 impl StreamCore {
@@ -88,9 +98,13 @@ impl StreamCore {
             width,
             raw_source: String::new(),
             rendered_lines: Vec::new(),
+            rendered_line_end_source_bytes: Vec::new(),
             enqueued_stable_len: 0,
             emitted_stable_len: 0,
             cwd: cwd.to_path_buf(),
+            emitted_stable_source_bytes: 0,
+            emitted_stable_lines_at_source_byte: 0,
+            emitted_stable_lines_at_source_byte_total: 0,
         }
     }
 
@@ -120,13 +134,12 @@ impl StreamCore {
         if !remainder_source.is_empty() {
             self.raw_source.push_str(&remainder_source);
         }
-        let mut rendered = Vec::new();
-        append_markdown_agent_with_cwd(
+        let rendered = render_markdown_agent_with_source_map_with_cwd(
             &self.raw_source,
             self.width,
             Some(self.cwd.as_path()),
-            &mut rendered,
         );
+        let rendered = rendered.lines;
         if self.emitted_stable_len >= rendered.len() {
             Vec::new()
         } else {
@@ -137,14 +150,18 @@ impl StreamCore {
     /// Step animation: dequeue one line, update the emitted count.
     fn tick(&mut self) -> Vec<Line<'static>> {
         let step = self.state.step();
+        let emitted_start = self.emitted_stable_len;
         self.emitted_stable_len += step.len();
+        self.advance_emitted_source_boundary(emitted_start, self.emitted_stable_len);
         step
     }
 
     /// Batch drain: dequeue up to `max_lines`, update the emitted count.
     fn tick_batch(&mut self, max_lines: usize) -> Vec<Line<'static>> {
         let step = self.state.drain_n(max_lines.max(1));
+        let emitted_start = self.emitted_stable_len;
         self.emitted_stable_len += step.len();
+        self.advance_emitted_source_boundary(emitted_start, self.emitted_stable_len);
         step
     }
 
@@ -171,41 +188,22 @@ impl StreamCore {
 
     /// Update rendering width and rebuild queued stable lines for the new layout.
     ///
-    /// When `emitted_stable_len > 0`, uses `source_bytes_for_rendered_count` to
-    /// find the byte offset in `raw_source` that corresponds to the old emitted
-    /// count, then re-renders that prefix at the new width to recalculate
-    /// `emitted_stable_len`. The stable queue is rebuilt from the new render.
+    /// Re-renders once at the new width and remaps already-emitted progress by
+    /// source boundary (`emitted_stable_source_bytes`) plus tie-break count for
+    /// wrapped lines that share the same line-end source offset.
     fn set_width(&mut self, width: Option<usize>) {
         if self.width == width {
             return;
         }
-        let old_width = self.width;
         self.width = width;
         self.state.collector.set_width(width);
         if self.raw_source.is_empty() {
             return;
         }
 
-        // Recalculate emitted_stabe_len for the new width so we don't re-queue lines that were
-        // already emitted at the old width.
-        if self.emitted_stable_len > 0 {
-            let emitted_bytes = source_bytes_for_rendered_count(
-                &self.raw_source,
-                old_width,
-                self.cwd.as_path(),
-                self.emitted_stable_len,
-            );
-            let mut emitted_at_new = Vec::new();
-            append_markdown_agent_with_cwd(
-                &self.raw_source[..emitted_bytes],
-                self.width,
-                Some(self.cwd.as_path()),
-                &mut emitted_at_new,
-            );
-            self.emitted_stable_len = emitted_at_new.len();
-        }
-
         self.recompute_streaming_render();
+        self.emitted_stable_len = self.remap_emitted_len_from_source_boundary();
+        self.sync_emitted_source_counters_from_render();
         self.rebuild_stable_queue_from_render();
     }
 
@@ -214,20 +212,27 @@ impl StreamCore {
         self.state.clear();
         self.raw_source.clear();
         self.rendered_lines.clear();
+        self.rendered_line_end_source_bytes.clear();
         self.enqueued_stable_len = 0;
         self.emitted_stable_len = 0;
+        self.emitted_stable_source_bytes = 0;
+        self.emitted_stable_lines_at_source_byte = 0;
+        self.emitted_stable_lines_at_source_byte_total = 0;
     }
 
     /// Re-render the full `raw_source` at current `width`.
     fn recompute_streaming_render(&mut self) {
-        let mut rendered = Vec::new();
-        append_markdown_agent_with_cwd(
+        let rendered = render_markdown_agent_with_source_map_with_cwd(
             &self.raw_source,
             self.width,
             Some(self.cwd.as_path()),
-            &mut rendered,
         );
-        self.rendered_lines = rendered;
+        self.rendered_lines = rendered.lines;
+        self.rendered_line_end_source_bytes = rendered.line_end_source_bytes;
+        debug_assert_eq!(
+            self.rendered_line_end_source_bytes.len(),
+            self.rendered_lines.len()
+        );
     }
 
     /// Advance `enqueued_stable_len` toward the target stable boundary and enqueue any
@@ -316,6 +321,87 @@ impl StreamCore {
             .len()
             .saturating_sub(stable_prefix_render.len())
     }
+
+    fn advance_emitted_source_boundary(&mut self, emitted_start: usize, emitted_end: usize) {
+        if emitted_start >= emitted_end || self.rendered_line_end_source_bytes.is_empty() {
+            return;
+        }
+
+        let end = emitted_end.min(self.rendered_line_end_source_bytes.len());
+        for line_end_source in &self.rendered_line_end_source_bytes[emitted_start..end] {
+            if *line_end_source > self.emitted_stable_source_bytes {
+                self.emitted_stable_source_bytes = *line_end_source;
+                self.emitted_stable_lines_at_source_byte = 1;
+                self.emitted_stable_lines_at_source_byte_total = self
+                    .rendered_line_end_source_bytes
+                    .iter()
+                    .filter(|source| **source == *line_end_source)
+                    .count()
+                    .max(1);
+            } else if *line_end_source == self.emitted_stable_source_bytes {
+                self.emitted_stable_lines_at_source_byte += 1;
+            }
+        }
+    }
+
+    fn remap_emitted_len_from_source_boundary(&self) -> usize {
+        if self.emitted_stable_len == 0 {
+            return 0;
+        }
+        if self.emitted_stable_source_bytes >= self.raw_source.len() {
+            return self.rendered_lines.len();
+        }
+
+        let mut lines_before = 0usize;
+        let mut lines_at = 0usize;
+        for line_end_source in &self.rendered_line_end_source_bytes {
+            if *line_end_source < self.emitted_stable_source_bytes {
+                lines_before += 1;
+            } else if *line_end_source == self.emitted_stable_source_bytes {
+                lines_at += 1;
+            }
+        }
+        let emitted_at_boundary = if self.emitted_stable_lines_at_source_byte
+            >= self.emitted_stable_lines_at_source_byte_total
+        {
+            lines_at
+        } else if lines_at >= self.emitted_stable_lines_at_source_byte {
+            self.emitted_stable_lines_at_source_byte
+        } else {
+            0
+        };
+        lines_before
+            + emitted_at_boundary.min(self.rendered_lines.len().saturating_sub(lines_before))
+    }
+
+    fn sync_emitted_source_counters_from_render(&mut self) {
+        let emitted_len = self
+            .emitted_stable_len
+            .min(self.rendered_line_end_source_bytes.len());
+        if emitted_len == 0 {
+            self.emitted_stable_source_bytes = 0;
+            self.emitted_stable_lines_at_source_byte = 0;
+            self.emitted_stable_lines_at_source_byte_total = 0;
+            return;
+        }
+
+        let boundary = self.rendered_line_end_source_bytes[emitted_len - 1];
+        let emitted_at_boundary = self.rendered_line_end_source_bytes[..emitted_len]
+            .iter()
+            .rev()
+            .take_while(|line_end| **line_end == boundary)
+            .count();
+        let total_at_boundary = self
+            .rendered_line_end_source_bytes
+            .iter()
+            .filter(|line_end| **line_end == boundary)
+            .count()
+            .max(1);
+
+        self.emitted_stable_source_bytes = boundary;
+        self.emitted_stable_lines_at_source_byte = emitted_at_boundary;
+        self.emitted_stable_lines_at_source_byte_total = total_at_boundary;
+    }
 }
 
 /// Controller for streaming agent message content with table-aware holdback.
@@ -512,53 +598,6 @@ impl PlanStreamController {
     }
 }
 
-// ---------------------------------------------------------------------------
-// source_bytes_for_rendered_count — resize remapping helper
-// ---------------------------------------------------------------------------
-
-/// Map a rendered-line count back to a source-byte offset for resize remapping.
-///
-/// Iterates through each newline in `raw_source`, rendering the prefix up to
-/// that newline at `width`, and returns the largest byte offset whose rendering
-/// produces at most `target_count` lines.
-///
-/// When the target falls exactly on a source-line boundary, the returned offset
-/// covers that line. When it falls in the middle of a wrapped source line
-/// (partial drain), the offset stops at the *previous* newline to avoid
-/// overshooting -- this may re-queue a few already-emitted wrapped lines as
-/// duplicates, but never drops un-emitted content.
-///
-/// This function is only called during `set_width` when `emitted_stable_len > 0`
-/// (i.e. non-table content has already been committed to scrollback).  For
-/// non-table content, rendering a newline-terminated prefix produces a prefix of
-/// the full rendering, so this converges correctly.
-///
-/// Performance: O(n) in source lines, each incurring a full markdown render of
-/// the prefix. Acceptable for typical agent message lengths (hundreds of lines).
-fn source_bytes_for_rendered_count(
-    raw_source: &str,
-    width: Option<usize>,
-    cwd: &Path,
-    target_count: usize,
-) -> usize {
-    if target_count == 0 {
-        return 0;
-    }
-    let mut best_offset = 0;
-    for (i, _) in raw_source.match_indices('\n') {
-        let prefix = &raw_source[..=i];
-        let mut lines = Vec::new();
-        append_markdown_agent_with_cwd(prefix, width, Some(cwd), &mut lines);
-        if lines.len() <= target_count {
-            best_offset = i + 1;
-        }
-        if lines.len() >= target_count {
-            break;
-        }
-    }
-    best_offset
-}
-
 // ---------------------------------------------------------------------------
 // Table holdback infrastructure
 // ---------------------------------------------------------------------------
@@ -769,6 +808,7 @@ fn table_holdback_state(source: &str) -> TableHoldbackState {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use pretty_assertions::assert_eq;
     use std::path::PathBuf;
 
     fn test_cwd() -> PathBuf {
@@ -1541,81 +1581,62 @@ mod tests {
         );
     }
 
-    // -----------------------------------------------------------------------
-    // source_bytes_for_rendered_count — prefix-stability tests
-    // -----------------------------------------------------------------------
-
     #[test]
-    fn source_bytes_plain_paragraphs() {
-        let src = "Alpha\n\nBravo\n\nCharlie\n";
-        let width = Some(80);
-        let cwd = test_cwd();
-
-        let mut full = Vec::new();
-        crate::markdown::append_markdown_agent(src, width, &mut full);
-        let total = full.len();
-        assert!(
-            total >= 3,
-            "expected at least 3 rendered lines, got {total}"
+    fn controller_set_width_after_first_line_emit_does_not_requeue_first_line() {
+        let mut ctrl = stream_controller(Some(120));
+        ctrl.push(
+            "FIRSTTOKEN contains enough words to wrap once the width is reduced dramatically.\n",
         );
+        ctrl.push("second line remains pending\n");
 
-        assert_eq!(source_bytes_for_rendered_count(src, width, &cwd, 0), 0);
-        assert_eq!(
-            source_bytes_for_rendered_count(src, width, &cwd, total),
-            src.len()
-        );
+        let (first_emit, _) = ctrl.on_commit_tick();
+        assert!(first_emit.is_some(), "expected first line emission");
 
-        let off = source_bytes_for_rendered_count(src, width, &cwd, 1);
-        assert!(off > 0 && off <= src.len());
-        let mut partial = Vec::new();
-        crate::markdown::append_markdown_agent(&src[..off], width, &mut partial);
+        ctrl.set_width(Some(20));
+
+        let (cell, _source) = ctrl.finalize();
+        let remaining = cell
+            .map(|cell| lines_to_plain_strings(&cell.transcript_lines(u16::MAX)))
+            .unwrap_or_default()
+            .into_iter()
+            .map(|line| line.chars().skip(2).collect::<String>())
+            .collect::<Vec<_>>();
         assert!(
-            partial.len() <= 1,
-            "expected <=1 line, got {}",
-            partial.len()
+            !remaining.iter().any(|line| line.contains("FIRSTTOKEN")),
+            "first line should not be re-queued after resize: {remaining:?}",
+        );
+        assert!(
+            remaining.iter().any(|line| line.contains("second line")),
+            "expected pending second line after resize: {remaining:?}",
         );
     }
 
     #[test]
-    fn source_bytes_wrapped_lines() {
-        let src = "The quick brown fox jumps over the lazy dog near the riverbank.\n";
-        let width = Some(20);
-        let cwd = test_cwd();
+    fn controller_set_width_partial_wrapped_emit_preserves_remaining_content() {
+        let mut ctrl = stream_controller(Some(20));
+        ctrl.push("The quick brown fox jumps over the lazy dog near the riverbank.\n");
+        ctrl.push("tail line\n");
 
-        let mut full = Vec::new();
-        crate::markdown::append_markdown_agent(src, width, &mut full);
-        let total = full.len();
+        let (first_emit, idle) = ctrl.on_commit_tick();
+        assert!(first_emit.is_some(), "expected first wrapped line emission");
+        assert!(!idle, "expected remaining queued content after one tick");
         assert!(
-            total > 1,
-            "expected wrapping at width 20, got {total} lines"
+            ctrl.queued_lines() > 0,
+            "expected non-empty queue before resize"
         );
 
-        let off = source_bytes_for_rendered_count(src, width, &cwd, 1);
-        assert_eq!(off, 0, "single wrapped source line should not be split");
-    }
+        ctrl.set_width(Some(120));
 
-    #[test]
-    fn source_bytes_list_items() {
-        let src = "- First item\n- Second item\n- Third item\n";
-        let width = Some(80);
-        let cwd = test_cwd();
-
-        let mut full = Vec::new();
-        crate::markdown::append_markdown_agent(src, width, &mut full);
-        let total = full.len();
+        let (cell, _source) = ctrl.finalize();
+        let remaining = cell
+            .map(|c| lines_to_plain_strings(&c.transcript_lines(u16::MAX)))
+            .unwrap_or_default()
+            .into_iter()
+            .map(|line| line.chars().skip(2).collect::<String>())
+            .collect::<Vec<_>>();
         assert!(
-            total >= 3,
-            "expected at least 3 rendered lines, got {total}"
-        );
-
-        let off = source_bytes_for_rendered_count(src, width, &cwd, 2);
-        assert!(off > 0);
-        let mut partial = Vec::new();
-        crate::markdown::append_markdown_agent(&src[..off], width, &mut partial);
-        assert!(
-            partial.len() <= 2,
-            "expected <=2 lines from prefix, got {}",
-            partial.len()
+            remaining.iter().any(|line| line.contains("tail line")),
+            "un-emitted content should remain after resize remap: {remaining:?}",
         );
     }
 }