tui: align diff display by always showing sign char and keeping fixed gutter (#2353)

diff lines without a sign char were misaligned.

AGENTS.md

@@ -11,3 +11,19 @@ In the codex-rs folder where the rust code lives:
 Before finalizing a change to `codex-rs`, run `just fmt` (in `codex-rs` directory) to format the code and `just fix` (in `codex-rs` directory) to fix any linter issues in the code. Additionally, run the tests:
 1. Run the test for the specific project that was changed. For example, if changes were made in `codex-rs/tui`, run `cargo test -p codex-tui`.
 2. Once those pass, if any changes were made in common, core, or protocol, run the complete test suite with `cargo test --all-features`.
+
+## Snapshot tests
+
+This repo uses snapshot tests (via `insta`), especially in `codex-rs/tui`, to validate rendered output. When UI or text output changes intentionally, update the snapshots as follows:
+
+- Run tests to generate any updated snapshots:
+  - `cargo test -p codex-tui`
+- Check what’s pending:
+  - `cargo insta pending-snapshots -p codex-tui`
+- Review changes by reading the generated `*.snap.new` files directly in the repo, or preview a specific file:
+  - `cargo insta show -p codex-tui path/to/file.snap.new`
+- Only if you intend to accept all new snapshots in this crate, run:
+  - `cargo insta accept -p codex-tui`
+
+If you don’t have the tool:
+- `cargo install cargo-insta`

codex-rs/tui/src/diff_render.rs

@@ -269,9 +269,9 @@ fn push_wrapped_diff_line(
     let mut remaining_text: &str = text;
 
     // Reserve a fixed number of spaces after the line number so that content starts
-    // at a consistent column. The sign ("+"/"-") is rendered as part of the content
-    // and colored with the same foreground as the edited text, not as a separate
-    // dimmed column.
+    // at a consistent column. Content includes a 1-character diff sign prefix
+    // ("+"/"-" for inserts/deletes, or a space for context lines) so alignment
+    // stays consistent across all diff lines.
     let gap_after_ln = SPACES_AFTER_LINE_NUMBER.saturating_sub(ln_str.len());
     let first_prefix_cols = indent.len() + ln_str.len() + gap_after_ln;
     let cont_prefix_cols = indent.len() + ln_str.len() + gap_after_ln;
@@ -306,14 +306,10 @@ fn push_wrapped_diff_line(
             spans.push(RtSpan::raw(indent));
             spans.push(RtSpan::styled(ln_str.clone(), style_dim()));
             spans.push(RtSpan::raw(" ".repeat(gap_after_ln)));
-
-            // Prefix the content with the sign if it is an insertion or deletion, and color
-            // the sign and content with the same foreground as the edited text.
-            let display_chunk = match sign_opt {
-                Some(sign_char) => format!("{sign_char}{chunk}"),
-                None => chunk.to_string(),
-            };
-
+            // Always include a sign character at the start of the displayed chunk
+            // ('+' for insert, '-' for delete, ' ' for context) so gutters align.
+            let sign_char = sign_opt.unwrap_or(' ');
+            let display_chunk = format!("{sign_char}{chunk}");
             let content_span = match line_style {
                 Some(style) => RtSpan::styled(display_chunk, style),
                 None => RtSpan::raw(display_chunk),
@@ -326,8 +322,9 @@ fn push_wrapped_diff_line(
             lines.push(line);
             first = false;
         } else {
+            // Continuation lines keep a space for the sign column so content aligns
             let hang_prefix = format!(
-                "{indent}{}{}",
+                "{indent}{}{} ",
                 " ".repeat(ln_str.len()),
                 " ".repeat(gap_after_ln)
             );

codex-rs/tui/src/snapshots/codex_tui__diff_render__tests__update_details_with_rename.snap

@@ -1,13 +1,14 @@
 ---
 source: tui/src/diff_render.rs
+assertion_line: 380
 expression: terminal.backend()
 ---
 "proposed patch to 1 file (+1 -1)                                                "
 "  └ src/lib.rs → src/lib_new.rs (+1 -1)                                         "
-"    1     line one                                                              "
+"    1      line one                                                             "
 "    2     -line two                                                             "
 "    2     +line two changed                                                     "
-"    3     line three                                                            "
+"    3      line three                                                           "
 "                                                                                "
 "                                                                                "
 "                                                                                "

codex-rs/tui/src/snapshots/codex_tui__diff_render__tests__wrap_behavior_insert.snap

@@ -1,9 +1,10 @@
 ---
 source: tui/src/diff_render.rs
+assertion_line: 380
 expression: terminal.backend()
 ---
 "    1     +this is a very long line that should wrap across multiple terminal col         "
-"          umns and continue                                                               "
+"           umns and continue                                                              "
 "                                                                                          "
 "                                                                                          "
 "                                                                                          "