fix(diff): preserve collapsed EOF marker

crates/maestro-tui/src/app.rs

@@ -477,7 +477,7 @@ impl App {
                                 .state
                                 .lock()
                                 .expect("structured state mutex")
-                                .approval_failed(error.to_string());
+                                .approval_failed(&nonce, error.to_string());
                         }
                     }
                 }

crates/maestro-tui/src/structured_pane.rs

@@ -14,6 +14,8 @@ use crate::session_task::SessionState;
 use crate::structured_task::{offered_decisions, StructuredHandle, StructuredState};
 
 const DIFF_PREVIEW_LINES: usize = 3;
+const DIFF_CONTEXT_LINES: usize = 3;
+const LCS_CELL_LIMIT: usize = 250_000;
 
 #[derive(Debug, Clone)]
 pub struct StructuredPane {
@@ -322,8 +324,8 @@ fn tool_lines(
         },
     });
 
-    let content = tool_content_lines(row);
     if row.kind == TranscriptRowKind::ToolDiff && !expanded {
+        let content = tool_content_lines(row, Some(DIFF_PREVIEW_LINES + 1));
         let shown = content.len().min(DIFF_PREVIEW_LINES);
         for text in content.iter().take(shown) {
             push_wrapped(out, row, "  ", text.clone(), width, diff_tone(text), false);
@@ -340,6 +342,7 @@ fn tool_lines(
             );
         }
     } else if expanded {
+        let content = tool_content_lines(row, None);
         for text in content {
             push_wrapped(out, row, "  ", text.clone(), width, diff_tone(&text), false);
         }
@@ -448,7 +451,7 @@ fn tool_title(row: &TranscriptRow) -> (String, String) {
     (explicit.unwrap_or(fallback).to_string(), title.to_string())
 }
 
-fn tool_content_lines(row: &TranscriptRow) -> Vec<String> {
+fn tool_content_lines(row: &TranscriptRow, limit: Option<usize>) -> Vec<String> {
     let mut out = Vec::new();
     for block in row
         .data
@@ -457,6 +460,9 @@ fn tool_content_lines(row: &TranscriptRow) -> Vec<String> {
         .into_iter()
         .flatten()
     {
+        if limit.is_some_and(|limit| out.len() >= limit) {
+            break;
+        }
         match block.get("type").and_then(serde_json::Value::as_str) {
             Some("content") => {
                 let nested = block.get("content").cloned().unwrap_or_default();
@@ -466,7 +472,8 @@ fn tool_content_lines(row: &TranscriptRow) -> Vec<String> {
                     data: serde_json::json!({"content": nested}),
                 };
                 let text = content_text(&fake);
-                out.extend(text.lines().map(str::to_string));
+                let remaining = limit.map_or(usize::MAX, |limit| limit.saturating_sub(out.len()));
+                out.extend(text.lines().take(remaining).map(str::to_string));
             }
             Some("diff") => {
                 let path = block
@@ -474,11 +481,10 @@ fn tool_content_lines(row: &TranscriptRow) -> Vec<String> {
                     .and_then(serde_json::Value::as_str)
                     .unwrap_or("file");
                 out.push(format!("{path}:"));
-                if let Some(old) = block.get("oldText").and_then(serde_json::Value::as_str) {
+                let old = block.get("oldText").and_then(serde_json::Value::as_str);
-                    out.extend(old.lines().map(|line| format!("- {line}")));
-                }
                 if let Some(new) = block.get("newText").and_then(serde_json::Value::as_str) {
-                    out.extend(new.lines().map(|line| format!("+ {line}")));
+                    let remaining = limit.map(|limit| limit.saturating_sub(out.len()));
+                    out.extend(render_diff(old, new, remaining));
                 }
             }
             Some("terminal") => out.push("[embedded terminal]".into()),
@@ -488,6 +494,236 @@ fn tool_content_lines(row: &TranscriptRow) -> Vec<String> {
     out
 }
 
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+struct SourceLine<'a> {
+    text: &'a str,
+    terminated: bool,
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+enum DiffKind {
+    Add,
+    Delete,
+    Context,
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+struct DiffOp<'a> {
+    kind: DiffKind,
+    line: SourceLine<'a>,
+}
+
+fn split_diff_lines(text: &str) -> Vec<SourceLine<'_>> {
+    text.split_inclusive('\n')
+        .map(|part| SourceLine {
+            text: part.strip_suffix('\n').unwrap_or(part),
+            terminated: part.ends_with('\n'),
+        })
+        .collect()
+}
+
+fn lcs_diff<'a>(old: &[SourceLine<'a>], new: &[SourceLine<'a>]) -> Vec<DiffOp<'a>> {
+    if old.is_empty() {
+        return new
+            .iter()
+            .copied()
+            .map(|line| DiffOp {
+                kind: DiffKind::Add,
+                line,
+            })
+            .collect();
+    }
+    if new.is_empty() {
+        return old
+            .iter()
+            .copied()
+            .map(|line| DiffOp {
+                kind: DiffKind::Delete,
+                line,
+            })
+            .collect();
+    }
+    if old.len().saturating_mul(new.len()) > LCS_CELL_LIMIT {
+        return old
+            .iter()
+            .copied()
+            .map(|line| DiffOp {
+                kind: DiffKind::Delete,
+                line,
+            })
+            .chain(new.iter().copied().map(|line| DiffOp {
+                kind: DiffKind::Add,
+                line,
+            }))
+            .collect();
+    }
+
+    let columns = new.len() + 1;
+    let mut table = vec![0_u32; (old.len() + 1) * columns];
+    for i in (0..old.len()).rev() {
+        for j in (0..new.len()).rev() {
+            table[i * columns + j] = if old[i] == new[j] {
+                table[(i + 1) * columns + j + 1] + 1
+            } else {
+                table[(i + 1) * columns + j].max(table[i * columns + j + 1])
+            };
+        }
+    }
+
+    let mut ops = Vec::with_capacity(old.len() + new.len());
+    let (mut i, mut j) = (0, 0);
+    while i < old.len() && j < new.len() {
+        if old[i] == new[j] {
+            ops.push(DiffOp {
+                kind: DiffKind::Context,
+                line: old[i],
+            });
+            i += 1;
+            j += 1;
+        } else if table[(i + 1) * columns + j] >= table[i * columns + j + 1] {
+            ops.push(DiffOp {
+                kind: DiffKind::Delete,
+                line: old[i],
+            });
+            i += 1;
+        } else {
+            ops.push(DiffOp {
+                kind: DiffKind::Add,
+                line: new[j],
+            });
+            j += 1;
+        }
+    }
+    ops.extend(old[i..].iter().copied().map(|line| DiffOp {
+        kind: DiffKind::Delete,
+        line,
+    }));
+    ops.extend(new[j..].iter().copied().map(|line| DiffOp {
+        kind: DiffKind::Add,
+        line,
+    }));
+    ops
+}
+
+fn diff_ops<'a>(old: &'a str, new: &'a str) -> Vec<DiffOp<'a>> {
+    let old = split_diff_lines(old);
+    let new = split_diff_lines(new);
+    let mut prefix = 0;
+    while prefix < old.len() && prefix < new.len() && old[prefix] == new[prefix] {
+        prefix += 1;
+    }
+    let mut suffix = 0;
+    while suffix < old.len() - prefix
+        && suffix < new.len() - prefix
+        && old[old.len() - 1 - suffix] == new[new.len() - 1 - suffix]
+    {
+        suffix += 1;
+    }
+
+    let mut ops = old[..prefix]
+        .iter()
+        .copied()
+        .map(|line| DiffOp {
+            kind: DiffKind::Context,
+            line,
+        })
+        .collect::<Vec<_>>();
+    ops.extend(lcs_diff(
+        &old[prefix..old.len() - suffix],
+        &new[prefix..new.len() - suffix],
+    ));
+    ops.extend(
+        old[old.len() - suffix..]
+            .iter()
+            .copied()
+            .map(|line| DiffOp {
+                kind: DiffKind::Context,
+                line,
+            }),
+    );
+    ops
+}
+
+fn render_diff_ops(ops: &[DiffOp<'_>]) -> Vec<String> {
+    let mut rendered = Vec::with_capacity(ops.len());
+    let mut index = 0;
+    while index < ops.len() {
+        if ops[index].kind != DiffKind::Context {
+            push_diff_op(&mut rendered, ops[index]);
+            index += 1;
+            continue;
+        }
+
+        let mut end = index;
+        while end < ops.len() && ops[end].kind == DiffKind::Context {
+            end += 1;
+        }
+        let run = &ops[index..end];
+        let whole_diff = index == 0 && end == ops.len();
+        let keep_head = if whole_diff || index > 0 {
+            DIFF_CONTEXT_LINES
+        } else {
+            0
+        };
+        let keep_tail = if whole_diff || end < ops.len() {
+            DIFF_CONTEXT_LINES
+        } else {
+            0
+        };
+        if run.len() <= keep_head + keep_tail {
+            for op in run {
+                push_diff_op(&mut rendered, *op);
+            }
+        } else {
+            for op in run.iter().take(keep_head) {
+                push_diff_op(&mut rendered, *op);
+            }
+            let collapsed = &run[keep_head..run.len() - keep_tail];
+            rendered.push(format!("… {} unchanged lines …", collapsed.len()));
+            if collapsed.iter().any(|op| !op.line.terminated) {
+                rendered.push("\\ No newline at end of file".into());
+            }
+            for op in run.iter().skip(run.len() - keep_tail) {
+                push_diff_op(&mut rendered, *op);
+            }
+        }
+        index = end;
+    }
+    rendered
+}
+
+fn push_diff_op(rendered: &mut Vec<String>, op: DiffOp<'_>) {
+    let prefix = match op.kind {
+        DiffKind::Add => "+ ",
+        DiffKind::Delete => "- ",
+        DiffKind::Context => "  ",
+    };
+    rendered.push(format!("{prefix}{}", op.line.text));
+    if !op.line.terminated {
+        rendered.push("\\ No newline at end of file".into());
+    }
+}
+
+fn render_diff(old: Option<&str>, new: &str, limit: Option<usize>) -> Vec<String> {
+    let ops = old.map_or_else(
+        || {
+            split_diff_lines(new)
+                .into_iter()
+                .map(|line| DiffOp {
+                    kind: DiffKind::Add,
+                    line,
+                })
+                .collect()
+        },
+        |old| diff_ops(old, new),
+    );
+    let mut rendered = render_diff_ops(&ops);
+    if let Some(limit) = limit {
+        rendered.truncate(limit);
+    }
+    rendered
+}
+
 fn diff_tone(text: &str) -> Tone {
     if text.starts_with("+ ") {
         Tone::Active
@@ -722,6 +958,230 @@ mod tests {
         assert!(lines.last().unwrap().text.contains("Enter to expand"));
     }
 
+    fn diff_content(old_text: Option<&str>, new_text: &str) -> Vec<String> {
+        let old_text = old_text.map_or(serde_json::Value::Null, |text| json!(text));
+        let row = row(
+            "d",
+            TranscriptRowKind::ToolDiff,
+            json!({"content":[{"type":"diff","path":"x","oldText":old_text,"newText":new_text}]}),
+        );
+        tool_content_lines(&row, None)
+    }
+
+    #[test]
+    fn structured_diff_preserves_unchanged_lines() {
+        assert_eq!(diff_content(Some("a\nb\n"), "a\nb\n"), ["x:", "  a", "  b"]);
+    }
+
+    #[test]
+    fn structured_diff_handles_insert_delete_replace_and_empty_files() {
+        assert_eq!(
+            diff_content(Some("a\nc\n"), "a\nb\nc\n"),
+            ["x:", "  a", "+ b", "  c"]
+        );
+        assert_eq!(
+            diff_content(Some("a\nb\nc\n"), "a\nc\n"),
+            ["x:", "  a", "- b", "  c"]
+        );
+        assert_eq!(
+            diff_content(Some("a\nb\n"), "a\nc\n"),
+            ["x:", "  a", "- b", "+ c"]
+        );
+        assert_eq!(diff_content(Some(""), ""), ["x:"]);
+        assert_eq!(diff_content(None, "a\n"), ["x:", "+ a"]);
+        assert_eq!(diff_content(Some("a\n"), ""), ["x:", "- a"]);
+    }
+
+    #[test]
+    fn structured_diff_marks_added_and_removed_final_newlines() {
+        assert_eq!(
+            diff_content(Some("a"), "a\n"),
+            ["x:", "- a", "\\ No newline at end of file", "+ a"]
+        );
+        assert_eq!(
+            diff_content(Some("a\n"), "a"),
+            ["x:", "- a", "+ a", "\\ No newline at end of file"]
+        );
+        assert_eq!(
+            diff_content(None, "a"),
+            ["x:", "+ a", "\\ No newline at end of file"]
+        );
+    }
+
+    #[test]
+    fn structured_diff_marks_unchanged_unterminated_final_context() {
+        assert_eq!(
+            diff_content(Some("a\nb\nc"), "a\nX\nc"),
+            [
+                "x:",
+                "  a",
+                "- b",
+                "+ X",
+                "  c",
+                "\\ No newline at end of file"
+            ]
+        );
+    }
+
+    #[test]
+    fn structured_diff_collapses_long_context_around_changes() {
+        let common = (0..20)
+            .map(|index| format!("line{index}"))
+            .collect::<Vec<_>>();
+        let old = common[..10]
+            .iter()
+            .cloned()
+            .chain(["OLD".into()])
+            .chain(common[10..].iter().cloned())
+            .collect::<Vec<_>>()
+            .join("\n");
+        let new = common[..10]
+            .iter()
+            .cloned()
+            .chain(["NEW".into()])
+            .chain(common[10..].iter().cloned())
+            .collect::<Vec<_>>()
+            .join("\n");
+
+        assert_eq!(
+            render_diff(Some(&old), &new, None),
+            [
+                "… 7 unchanged lines …",
+                "  line7",
+                "  line8",
+                "  line9",
+                "- OLD",
+                "+ NEW",
+                "  line10",
+                "  line11",
+                "  line12",
+                "… 7 unchanged lines …",
+                "\\ No newline at end of file",
+            ]
+        );
+    }
+
+    #[test]
+    fn structured_diff_preserves_eof_marker_through_trailing_collapse() {
+        let trailing = (0..10).map(|index| format!("t{index}")).collect::<Vec<_>>();
+        let old = ["OLD".into()]
+            .into_iter()
+            .chain(trailing.iter().cloned())
+            .collect::<Vec<_>>()
+            .join("\n");
+        let new = ["NEW".into()]
+            .into_iter()
+            .chain(trailing)
+            .collect::<Vec<_>>()
+            .join("\n");
+
+        assert_eq!(
+            render_diff(Some(&old), &new, None),
+            [
+                "- OLD",
+                "+ NEW",
+                "  t0",
+                "  t1",
+                "  t2",
+                "… 7 unchanged lines …",
+                "\\ No newline at end of file",
+            ]
+        );
+    }
+
+    #[test]
+    fn structured_diff_collapses_only_long_interior_context() {
+        let middle = (0..10).map(|index| format!("c{index}")).collect::<Vec<_>>();
+        let old = ["lead0", "lead1", "OLD1"]
+            .into_iter()
+            .map(str::to_string)
+            .chain(middle.iter().cloned())
+            .chain(["OLD2", "trail0", "trail1"].into_iter().map(str::to_string))
+            .collect::<Vec<_>>()
+            .join("\n");
+        let new = ["lead0", "lead1", "NEW1"]
+            .into_iter()
+            .map(str::to_string)
+            .chain(middle.iter().cloned())
+            .chain(["NEW2", "trail0", "trail1"].into_iter().map(str::to_string))
+            .collect::<Vec<_>>()
+            .join("\n");
+        let rendered = render_diff(Some(&old), &new, None);
+
+        assert_eq!(
+            rendered
+                .iter()
+                .filter(|line| line.contains("unchanged lines"))
+                .count(),
+            1
+        );
+        assert_eq!(&rendered[..4], ["  lead0", "  lead1", "- OLD1", "+ NEW1"]);
+        assert_eq!(
+            &rendered[4..11],
+            [
+                "  c0",
+                "  c1",
+                "  c2",
+                "… 4 unchanged lines …",
+                "  c7",
+                "  c8",
+                "  c9"
+            ]
+        );
+        assert_eq!(
+            &rendered[11..],
+            [
+                "- OLD2",
+                "+ NEW2",
+                "  trail0",
+                "  trail1",
+                "\\ No newline at end of file"
+            ]
+        );
+    }
+
+    #[test]
+    fn structured_diff_preview_remains_bounded_after_hunking() {
+        let common = (0..10)
+            .map(|index| format!("line{index}"))
+            .collect::<Vec<_>>();
+        let old = common
+            .iter()
+            .cloned()
+            .chain(["OLD".into()])
+            .collect::<Vec<_>>()
+            .join("\n");
+        let new = common
+            .iter()
+            .cloned()
+            .chain(["NEW".into()])
+            .collect::<Vec<_>>()
+            .join("\n");
+        let expanded = render_diff(Some(&old), &new, None);
+
+        assert_eq!(render_diff(Some(&old), &new, Some(4)), expanded[..4]);
+        assert_eq!(expanded[0], "… 7 unchanged lines …");
+        assert!(expanded.contains(&"- OLD".into()));
+        assert!(expanded.contains(&"+ NEW".into()));
+    }
+
+    #[test]
+    fn structured_diff_bounds_pathological_changed_middles() {
+        let old_lines = (0..501)
+            .map(|index| format!("line{index}"))
+            .collect::<Vec<_>>();
+        let mut new_lines = old_lines[1..].to_vec();
+        new_lines.push(old_lines[0].clone());
+        let old = format!("{}\n", old_lines.join("\n"));
+        let new = format!("{}\n", new_lines.join("\n"));
+        let rendered = render_diff(Some(&old), &new, None);
+
+        assert_eq!(rendered.len(), 1_002);
+        assert!(rendered[..501].iter().all(|line| line.starts_with("- ")));
+        assert!(rendered[501..].iter().all(|line| line.starts_with("+ ")));
+        assert_eq!(render_diff(Some(&old), &new, Some(4)), rendered[..4]);
+    }
+
     #[test]
     fn plan_is_not_a_transcript_row_and_status_is_two_lines() {
         let mut state = StructuredState::default();

crates/maestro-tui/src/structured_task.rs

@@ -289,7 +289,10 @@ impl StructuredState {
         Some((nonce, decision))
     }
 
-    pub fn approval_failed(&mut self, error: String) {
+    pub fn approval_failed(&mut self, nonce: &str, error: String) {
+        if self.pending_nonce() != Some(nonce) {
+            return;
+        }
         self.approval_submitting = false;
         self.error = Some(error);
     }
@@ -554,6 +557,81 @@ mod tests {
         );
     }
 
+    #[test]
+    fn resolved_approval_ignores_its_late_failure() {
+        let mut state = StructuredState::default();
+        state.rows.push(row(
+            "approval:1",
+            TranscriptRowKind::Approval,
+            json!({"options":[{"kind":"allow_once"}]}),
+        ));
+        state.selected = Some("approval:1".into());
+        state.reduced.pending_approval = Some(json!({"nonce":"old"}));
+        assert!(state.approval_decision('a').is_some());
+
+        state.apply(StructuredEvent::ProjectionUpdate {
+            transcript: None,
+            state: ReducedState::default(),
+        });
+        state.approval_failed("old", "response lost".into());
+
+        assert_eq!(state.error, None);
+        assert!(!state.approval_submitting);
+    }
+
+    #[test]
+    fn superseded_approval_ignores_its_late_failure() {
+        let mut state = StructuredState::default();
+        state.rows.push(row(
+            "approval:1",
+            TranscriptRowKind::Approval,
+            json!({"options":[{"kind":"allow_once"}]}),
+        ));
+        state.selected = Some("approval:1".into());
+        state.reduced.pending_approval = Some(json!({"nonce":"old"}));
+        assert!(state.approval_decision('a').is_some());
+
+        state.rows.push(row(
+            "approval:2",
+            TranscriptRowKind::Approval,
+            json!({"options":[{"kind":"allow_once"}]}),
+        ));
+        state.apply(StructuredEvent::ProjectionUpdate {
+            transcript: None,
+            state: ReducedState {
+                pending_approval: Some(json!({"nonce":"new"})),
+                ..ReducedState::default()
+            },
+        });
+        assert_eq!(
+            state.approval_decision('a'),
+            Some(("new".into(), StructuredApprovalDecision::AllowOnce))
+        );
+
+        state.approval_failed("old", "response lost".into());
+
+        assert_eq!(state.error, None);
+        assert!(state.approval_submitting);
+    }
+
+    #[test]
+    fn matching_approval_surfaces_its_failure() {
+        let mut state = StructuredState::default();
+        state.rows.push(row(
+            "approval:1",
+            TranscriptRowKind::Approval,
+            json!({"options":[{"kind":"allow_once"}]}),
+        ));
+        state.selected = Some("approval:1".into());
+        state.reduced.pending_approval = Some(json!({"nonce":"current"}));
+        assert!(state.approval_decision('a').is_some());
+
+        state.approval_failed("current", "request failed".into());
+
+        assert_eq!(state.error.as_deref(), Some("request failed"));
+        assert!(!state.approval_submitting);
+    }
+
     #[test]
     fn composer_is_gated_by_the_projected_turn_lifecycle() {
         let mut state = StructuredState::default();

web/src/structured.test.ts

@@ -115,6 +115,113 @@ describe("structured connection: data flow", () => {
     expect(conn.el.textContent).not.toContain("partial\n"); // the un-patched text is gone, not appended beside it
     conn.dispose();
   });
+
+  it("keeps an expanded running tool open without emitting a click when patched", () => {
+    const ws = new FakeSocket();
+    const conn = mount({ sockets: [ws] });
+    ws.open();
+    ws.emit({
+      type: "transcript_snapshot",
+      snapshot: {
+        cursor: 1,
+        rows: [
+          row("tool:1", "tool_call", {
+            title: "Bash(pnpm test)",
+            status: "in_progress",
+            content: [{ type: "content", content: { type: "text", text: "starting" } }],
+          }),
+        ],
+      },
+    });
+    ws.emit({ type: "reduced_state", state: idleState });
+
+    const originalHead = conn.el.querySelector<HTMLButtonElement>(".st-tool-head")!;
+    originalHead.click();
+    expect(originalHead.getAttribute("aria-expanded")).toBe("true");
+    const clickListener = vi.fn();
+    window.addEventListener("click", clickListener, { capture: true, once: true });
+
+    ws.emit({
+      type: "projection_update",
+      transcript: {
+        type: "patch",
+        row: row("tool:1", "tool_call", {
+          title: "Bash(pnpm test)",
+          status: "in_progress",
+          content: [{ type: "content", content: { type: "text", text: "still running" } }],
+        }),
+      },
+      state: idleState,
+    });
+
+    expect(clickListener).not.toHaveBeenCalled();
+    window.removeEventListener("click", clickListener, true);
+    const patchedHead = conn.el.querySelector<HTMLButtonElement>(".st-tool-head")!;
+    expect(patchedHead).not.toBe(originalHead);
+    expect(patchedHead.getAttribute("aria-expanded")).toBe("true");
+    expect(conn.el.querySelector<HTMLElement>(".st-tool-out")!.hidden).toBe(false);
+    expect(conn.el.textContent).toContain("still running");
+    conn.dispose();
+  });
+
+  it("restores focus to a patched tool header with the same stable row id", () => {
+    const ws = new FakeSocket();
+    const conn = mount({ sockets: [ws] });
+    ws.open();
+    ws.emit({
+      type: "transcript_snapshot",
+      snapshot: {
+        cursor: 1,
+        rows: [row("tool:1", "tool_call", { title: "Read(file.ts)", status: "in_progress" })],
+      },
+    });
+    ws.emit({ type: "reduced_state", state: idleState });
+    const originalHead = conn.el.querySelector<HTMLButtonElement>(".st-tool-head")!;
+    originalHead.focus();
+    expect(document.activeElement).toBe(originalHead);
+
+    ws.emit({
+      type: "projection_update",
+      transcript: {
+        type: "patch",
+        row: row("tool:1", "tool_call", { title: "Read(file.ts)", status: "completed" }),
+      },
+      state: idleState,
+    });
+
+    const patchedHead = conn.el.querySelector<HTMLButtonElement>(".st-tool-head")!;
+    expect(patchedHead).not.toBe(originalHead);
+    expect(document.activeElement).toBe(patchedHead);
+    conn.dispose();
+  });
+
+  it("does not move unrelated focus when a tool row is patched", () => {
+    const ws = new FakeSocket();
+    const conn = mount({ sockets: [ws] });
+    ws.open();
+    ws.emit({
+      type: "transcript_snapshot",
+      snapshot: {
+        cursor: 1,
+        rows: [row("tool:1", "tool_call", { title: "Read(file.ts)", status: "in_progress" })],
+      },
+    });
+    ws.emit({ type: "reduced_state", state: idleState });
+    const input = conn.el.querySelector<HTMLTextAreaElement>(".structured-input")!;
+    input.focus();
+
+    ws.emit({
+      type: "projection_update",
+      transcript: {
+        type: "patch",
+        row: row("tool:1", "tool_call", { title: "Read(file.ts)", status: "completed" }),
+      },
+      state: idleState,
+    });
+
+    expect(document.activeElement).toBe(input);
+    conn.dispose();
+  });
 });
 
 describe("structured connection: turn/composer gating", () => {

web/src/structured.ts

@@ -159,10 +159,15 @@ export function createStructuredConnection(
 
   function remountRow(row: TranscriptRow): void {
     const old = rowEls.get(row.id);
-    const el = buildTranscriptRow(row, approvalStateFor(row));
+    const oldToolHead = old?.querySelector<HTMLButtonElement>(".st-tool-head") ?? null;
+    const wasExpanded = oldToolHead?.getAttribute("aria-expanded") === "true";
+    const restoreFocus = document.activeElement === oldToolHead;
+    const el = buildTranscriptRow(row, approvalStateFor(row), wasExpanded);
     rowEls.set(row.id, el);
     if (old) old.replaceWith(el);
     else rowsEl.appendChild(el);
+    const newToolHead = el.querySelector<HTMLButtonElement>(".st-tool-head");
+    if (restoreFocus) newToolHead?.focus();
   }
 
   function applySnapshot(snapshot: { rows: TranscriptRow[] }): void {

web/src/structuredDiff.test.ts

@@ -42,6 +42,61 @@ describe("computeDiffLines", () => {
     ]);
   });
 
+  it("handles an insertion without changing surrounding lines", () => {
+    expect(texts(computeDiffLines("a\nc\n", "a\nb\nc\n"))).toEqual([
+      ["ctx", "a"],
+      ["add", "b"],
+      ["ctx", "c"],
+    ]);
+  });
+
+  it("handles a deletion without changing surrounding lines", () => {
+    expect(texts(computeDiffLines("a\nb\nc\n", "a\nc\n"))).toEqual([
+      ["ctx", "a"],
+      ["del", "b"],
+      ["ctx", "c"],
+    ]);
+  });
+
+  it("renders two empty files without invented lines", () => {
+    expect(computeDiffLines("", "")).toEqual([]);
+  });
+
+  it("shows when a final newline is added", () => {
+    expect(texts(computeDiffLines("a", "a\n"))).toEqual([
+      ["del", "a"],
+      ["eof", "\\ No newline at end of file"],
+      ["add", "a"],
+    ]);
+  });
+
+  it("shows when a final newline is removed", () => {
+    expect(texts(computeDiffLines("a\n", "a"))).toEqual([
+      ["del", "a"],
+      ["add", "a"],
+      ["eof", "\\ No newline at end of file"],
+    ]);
+  });
+
+  it("marks an unchanged unterminated final context line", () => {
+    expect(texts(computeDiffLines("a\nb\nc", "a\nX\nc"))).toEqual([
+      ["ctx", "a"],
+      ["del", "b"],
+      ["add", "X"],
+      ["ctx", "c"],
+      ["eof", "\\ No newline at end of file"],
+    ]);
+  });
+
+  it("marks an unterminated new file only once", () => {
+    const lines = computeDiffLines(null, "a");
+    expect(texts(lines)).toEqual([
+      ["add", "a"],
+      ["eof", "\\ No newline at end of file"],
+    ]);
+    expect(lines.filter((line) => line.kind === "eof")).toHaveLength(1);
+  });
+
   it("collapses a long unchanged interior run, keeping 3 lines of context each side", () => {
     const common = Array.from({ length: 20 }, (_, i) => `line${i}`);
     const oldText = [...common.slice(0, 10), "OLD", ...common.slice(10)].join("\n");
@@ -59,6 +114,22 @@ describe("computeDiffLines", () => {
     expect(lines.filter((l) => l.kind === "hunk")).toHaveLength(2);
   });
 
+  it("preserves the EOF marker when a long unterminated trailing run collapses", () => {
+    const trailing = Array.from({ length: 10 }, (_, i) => `t${i}`);
+    const oldText = ["OLD", ...trailing].join("\n");
+    const newText = ["NEW", ...trailing].join("\n");
+
+    expect(texts(computeDiffLines(oldText, newText))).toEqual([
+      ["del", "OLD"],
+      ["add", "NEW"],
+      ["ctx", "t0"],
+      ["ctx", "t1"],
+      ["ctx", "t2"],
+      ["hunk", "⋯ 7 unchanged lines ⋯"],
+      ["eof", "\\ No newline at end of file"],
+    ]);
+  });
+
   it("produces one hunk for the long middle run, none for the short edges", () => {
     // Edge runs (2 lines) fit within their one-sided 3-line context budget;
     // only the interior run (10 lines, budgeted 3+3=6) needs to collapse.
@@ -75,9 +146,10 @@ describe("computeDiffLines", () => {
       ["ctx", "lead0"],
       ["ctx", "lead1"],
     ]);
-    expect(texts(lines).slice(-2)).toEqual([
+    expect(texts(lines).slice(-3)).toEqual([
       ["ctx", "trail0"],
       ["ctx", "trail1"],
+      ["eof", "\\ No newline at end of file"],
     ]);
   });
 
@@ -85,10 +157,22 @@ describe("computeDiffLines", () => {
     const lines = computeDiffLines("a\nb", "a\nb\nc");
     expect(texts(lines)).toEqual([
       ["ctx", "a"],
-      ["ctx", "b"],
+      ["del", "b"],
+      ["eof", "\\ No newline at end of file"],
+      ["add", "b"],
       ["add", "c"],
+      ["eof", "\\ No newline at end of file"],
     ]);
   });
+
+  it("falls back to a bounded coarse diff for a pathological changed middle", () => {
+    const oldLines = Array.from({ length: 501 }, (_, index) => `line${index}`);
+    const newLines = [...oldLines.slice(1), oldLines[0]];
+    const lines = computeDiffLines(`${oldLines.join("\n")}\n`, `${newLines.join("\n")}\n`);
+    expect(lines).toHaveLength(1_002);
+    expect(lines.slice(0, 501).every((line) => line.kind === "del")).toBe(true);
+    expect(lines.slice(501).every((line) => line.kind === "add")).toBe(true);
+  });
 });
 
 describe("diffStats", () => {

web/src/structuredDiff.ts

@@ -2,7 +2,7 @@
 // hands over full `oldText`/`newText`, not a unified-diff patch, so the
 // coloured hunks the design calls for have to be computed here.
 
-export type DiffLineKind = "add" | "del" | "ctx" | "hunk";
+export type DiffLineKind = "add" | "del" | "ctx" | "hunk" | "eof";
 
 export interface DiffLine {
   kind: DiffLineKind;
@@ -19,66 +19,82 @@ const CONTEXT = 3;
  *  still correct, just not minimal. */
 const LCS_CELL_LIMIT = 250_000;
 
-function splitLines(text: string): string[] {
+interface SourceLine {
-  if (text === "") return [];
+  text: string;
-  const body = text.endsWith("\n") ? text.slice(0, -1) : text;
+  terminated: boolean;
-  return body.split("\n");
 }
 
-type Op = { kind: "add" | "del" | "ctx"; text: string };
+function splitLines(text: string): SourceLine[] {
+  if (text === "") return [];
+  const parts = text.split("\n");
+  const hasFinalNewline = text.endsWith("\n");
+  if (hasFinalNewline) parts.pop();
+  return parts.map((line, index) => ({
+    text: line,
+    terminated: index < parts.length - 1 || hasFinalNewline,
+  }));
+}
+
+type Op = { kind: "add" | "del" | "ctx"; line: SourceLine };
+
+function sameLine(a: SourceLine, b: SourceLine): boolean {
+  return a.text === b.text && a.terminated === b.terminated;
+}
 
 /** Longest-common-subsequence diff of two line arrays, already trimmed of any
  *  shared prefix/suffix by the caller. */
-function lcsDiff(a: string[], b: string[]): Op[] {
+function lcsDiff(a: SourceLine[], b: SourceLine[]): Op[] {
   const n = a.length;
   const m = b.length;
-  if (n === 0) return b.map((text) => ({ kind: "add", text }));
+  if (n === 0) return b.map((line) => ({ kind: "add", line }));
-  if (m === 0) return a.map((text) => ({ kind: "del", text }));
+  if (m === 0) return a.map((line) => ({ kind: "del", line }));
   if (n * m > LCS_CELL_LIMIT) {
-    return [...a.map((text) => ({ kind: "del" as const, text })), ...b.map((text) => ({ kind: "add" as const, text }))];
+    return [...a.map((line) => ({ kind: "del" as const, line })), ...b.map((line) => ({ kind: "add" as const, line }))];
   }
   // dp[i][j] = length of the LCS of a[i:] and b[j:].
-  const dp: Uint32Array[] = Array.from({ length: n + 1 }, () => new Uint32Array(m + 1));
+  const columns = m + 1;
+  const dp = new Uint32Array((n + 1) * columns);
   for (let i = n - 1; i >= 0; i--) {
     for (let j = m - 1; j >= 0; j--) {
-      dp[i][j] =
+      dp[i * columns + j] = sameLine(a[i], b[j])
-        a[i] === b[j] ? dp[i + 1][j + 1] + 1 : Math.max(dp[i + 1][j], dp[i][j + 1]);
+        ? dp[(i + 1) * columns + j + 1] + 1
+        : Math.max(dp[(i + 1) * columns + j], dp[i * columns + j + 1]);
     }
   }
   const ops: Op[] = [];
   let i = 0;
   let j = 0;
   while (i < n && j < m) {
-    if (a[i] === b[j]) {
+    if (sameLine(a[i], b[j])) {
-      ops.push({ kind: "ctx", text: a[i] });
+      ops.push({ kind: "ctx", line: a[i] });
       i++;
       j++;
-    } else if (dp[i + 1][j] >= dp[i][j + 1]) {
+    } else if (dp[(i + 1) * columns + j] >= dp[i * columns + j + 1]) {
-      ops.push({ kind: "del", text: a[i] });
+      ops.push({ kind: "del", line: a[i] });
       i++;
     } else {
-      ops.push({ kind: "add", text: b[j] });
+      ops.push({ kind: "add", line: b[j] });
       j++;
     }
   }
-  while (i < n) ops.push({ kind: "del", text: a[i++] });
+  while (i < n) ops.push({ kind: "del", line: a[i++] });
-  while (j < m) ops.push({ kind: "add", text: b[j++] });
+  while (j < m) ops.push({ kind: "add", line: b[j++] });
   return ops;
 }
 
-function diffLines(a: string[], b: string[]): Op[] {
+function diffLines(a: SourceLine[], b: SourceLine[]): Op[] {
   let pre = 0;
-  while (pre < a.length && pre < b.length && a[pre] === b[pre]) pre++;
+  while (pre < a.length && pre < b.length && sameLine(a[pre], b[pre])) pre++;
   let suf = 0;
   while (
     suf < a.length - pre &&
     suf < b.length - pre &&
-    a[a.length - 1 - suf] === b[b.length - 1 - suf]
+    sameLine(a[a.length - 1 - suf], b[b.length - 1 - suf])
   ) {
     suf++;
   }
-  const prefixCtx: Op[] = a.slice(0, pre).map((text) => ({ kind: "ctx", text }));
+  const prefixCtx: Op[] = a.slice(0, pre).map((line) => ({ kind: "ctx", line }));
-  const suffixCtx: Op[] = a.slice(a.length - suf).map((text) => ({ kind: "ctx", text }));
+  const suffixCtx: Op[] = a.slice(a.length - suf).map((line) => ({ kind: "ctx", line }));
   const middle = lcsDiff(a.slice(pre, a.length - suf), b.slice(pre, b.length - suf));
   return [...prefixCtx, ...middle, ...suffixCtx];
 }
@@ -91,10 +107,16 @@ function diffLines(a: string[], b: string[]): Op[] {
  *  (unless it is the *only* run, i.e. nothing changed at all). */
 function hunk(ops: Op[]): DiffLine[] {
   const out: DiffLine[] = [];
+  const push = (op: Op) => {
+    out.push({ kind: op.kind, text: op.line.text });
+    if (!op.line.terminated) {
+      out.push({ kind: "eof", text: "\\ No newline at end of file" });
+    }
+  };
   let i = 0;
   while (i < ops.length) {
     if (ops[i].kind !== "ctx") {
-      out.push(ops[i]);
+      push(ops[i]);
       i++;
       continue;
     }
@@ -105,14 +127,18 @@ function hunk(ops: Op[]): DiffLine[] {
     const keepHead = wholeDiff || i > 0 ? CONTEXT : 0;
     const keepTail = wholeDiff || j < ops.length ? CONTEXT : 0;
     if (run.length <= keepHead + keepTail) {
-      out.push(...run);
+      run.forEach(push);
     } else {
-      if (keepHead) out.push(...run.slice(0, keepHead));
+      if (keepHead) run.slice(0, keepHead).forEach(push);
+      const collapsed = run.slice(keepHead, run.length - keepTail);
       out.push({
         kind: "hunk",
-        text: `⋯ ${run.length - keepHead - keepTail} unchanged lines ⋯`,
+        text: `⋯ ${collapsed.length} unchanged lines ⋯`,
       });
-      if (keepTail) out.push(...run.slice(run.length - keepTail));
+      if (collapsed.some((op) => !op.line.terminated)) {
+        out.push({ kind: "eof", text: "\\ No newline at end of file" });
+      }
+      if (keepTail) run.slice(run.length - keepTail).forEach(push);
     }
     i = j;
   }
@@ -123,7 +149,7 @@ function hunk(ops: Op[]): DiffLine[] {
  *  (a new file) renders every line of `newText` as added. */
 export function computeDiffLines(oldText: string | null | undefined, newText: string): DiffLine[] {
   const newLines = splitLines(newText);
-  if (oldText == null) return newLines.map((text) => ({ kind: "add", text }));
+  if (oldText == null) return hunk(newLines.map((line) => ({ kind: "add", line })));
   const ops = diffLines(splitLines(oldText), newLines);
   return hunk(ops);
 }

web/src/structuredRender.test.ts

@@ -157,6 +157,17 @@ describe("buildTranscriptRow: tool diffs", () => {
     // stylesheet keys `overflow-x: auto` off, not a computed style.
     expect(el.querySelector(".st-diff")!.className).toBe("st-diff");
   });
+
+  it("renders a visible marker for a removed final newline", () => {
+    const el = buildTranscriptRow(
+      row("tool_diff", {
+        title: "Edit(x.rs)",
+        content: [{ type: "diff", path: "/repo/x.rs", oldText: "same\n", newText: "same" }],
+      }),
+    );
+    (el.querySelector(".st-tool-head") as HTMLButtonElement).click();
+    expect(el.querySelector(".st-ln-eof")!.textContent).toBe("\\ No newline at end of file");
+  });
 });
 
 describe("buildTranscriptRow: approval", () => {

web/src/structuredRender.ts

@@ -187,7 +187,7 @@ function statusMeta(status: string | undefined, hasDiff: boolean): { text: strin
   return null;
 }
 
-function buildToolRow(row: TranscriptRow): HTMLElement {
+function buildToolRow(row: TranscriptRow, expanded: boolean): HTMLElement {
   const tool = (row.data ?? {}) as AcpToolCallLike;
   const content = tool.content ?? [];
   const hasDiff = content.some((c) => c.type === "diff");
@@ -251,13 +251,16 @@ function buildToolRow(row: TranscriptRow): HTMLElement {
   };
   card.appendChild(out);
 
+  const setExpanded = (nextExpanded: boolean) => {
+    if (nextExpanded) buildOut();
+    if (out.children.length === 0) nextExpanded = false;
+    head.setAttribute("aria-expanded", String(nextExpanded));
+    out.hidden = !nextExpanded;
+  };
   head.addEventListener("click", () => {
-    const expanded = head.getAttribute("aria-expanded") === "true";
+    setExpanded(head.getAttribute("aria-expanded") !== "true");
-    if (!expanded) buildOut();
-    if (out.children.length === 0) return; // nothing to show — stay collapsed
-    head.setAttribute("aria-expanded", String(!expanded));
-    out.hidden = expanded;
   });
+  setExpanded(expanded);
 
   return wrap;
 }
@@ -361,7 +364,11 @@ function buildApprovalRow(row: TranscriptRow, approval: ApprovalRowState): HTMLE
 
 /** Build the DOM for one transcript row. `approval` is consulted only for an
  *  `approval` row; pass it whenever the row might be that kind. */
-export function buildTranscriptRow(row: TranscriptRow, approval?: ApprovalRowState): HTMLElement {
+export function buildTranscriptRow(
+  row: TranscriptRow,
+  approval?: ApprovalRowState,
+  toolExpanded = false,
+): HTMLElement {
   let built: HTMLElement;
   switch (row.kind) {
     case "user":
@@ -375,7 +382,7 @@ export function buildTranscriptRow(row: TranscriptRow, approval?: ApprovalRowSta
       break;
     case "tool_call":
     case "tool_diff":
-      built = buildToolRow(row);
+      built = buildToolRow(row, toolExpanded);
       break;
     case "approval":
       built = buildApprovalRow(row, approval ?? { pending: false, onDecide: () => Promise.resolve() });

web/src/styles.css

@@ -2509,6 +2509,11 @@ input.leaf-rename:focus {
   background: var(--surface-2);
 }
 
+.st-ln-eof {
+  color: var(--faint);
+  font-style: italic;
+}
+
 /* the one lifted, card-like element on the surface: the only thing the
    operator owes a decision on */
 .st-approval {