New option -U, --context-lines to solve incorrect multiline syntax highlighting

etc/examples/create-context-demo-repos.sh

@@ -0,0 +1,161 @@
+#!/usr/bin/env bash
+#
+# Creates three small git repos demonstrating the syntax-highlighting context
+# bug. In each repo, `git diff HEAD~1` produces a hunk that starts inside a
+# multiline construct (docstring / block comment / template literal), so
+# syntect misparses the closing delimiter.
+#
+# Usage:
+#   bash etc/examples/create-context-demo-repos.sh
+#
+# Then compare:
+#   cd /tmp/context-demo-python
+#   git diff HEAD~1 | delta                          # bug: return is string-colored
+#   git diff -U9999 HEAD~1 | delta -U 3              # fix: return is keyword-colored
+
+set -euo pipefail
+
+DIR="${1:-/tmp}"
+
+# --- Python triple-quoted string ---
+REPO="$DIR/context-demo-python"
+rm -rf "$REPO"
+mkdir -p "$REPO" && cd "$REPO" && git init -q
+
+cat > example.py << 'PYEOF'
+def foo():
+    """
+    This is a docstring that spans
+    multiple lines.
+    It has lots of content.
+    More content here.
+    Even more explanation.
+    Still going on.
+    And yet more.
+    Final docstring line.
+    """
+    x = 1
+    return x
+PYEOF
+git add example.py && git commit -q -m "initial"
+
+cat > example.py << 'PYEOF'
+def foo():
+    """
+    This is a docstring that spans
+    multiple lines.
+    It has lots of content.
+    More content here.
+    Even more explanation.
+    Still going on.
+    And yet more.
+    Final docstring line.
+    """
+    x = 2
+    return x + 1
+PYEOF
+git add example.py && git commit -q -m "change"
+
+# --- Rust block comment ---
+REPO="$DIR/context-demo-rust"
+rm -rf "$REPO"
+mkdir -p "$REPO" && cd "$REPO" && git init -q
+
+cat > lib.rs << 'RSEOF'
+/// Entry point for the library.
+pub fn process(input: &str) -> Result<String, Error> {
+    /*
+     * Multi-line comment explaining
+     * the complex validation logic
+     * that follows below.
+     * It covers edge cases for:
+     * - empty input
+     * - unicode input
+     * - oversized input
+     * - malformed input
+     */
+    let validated = validate(input)?;
+    let result = transform(validated);
+    Ok(result)
+}
+RSEOF
+git add lib.rs && git commit -q -m "initial"
+
+cat > lib.rs << 'RSEOF'
+/// Entry point for the library.
+pub fn process(input: &str) -> Result<String, Error> {
+    /*
+     * Multi-line comment explaining
+     * the complex validation logic
+     * that follows below.
+     * It covers edge cases for:
+     * - empty input
+     * - unicode input
+     * - oversized input
+     * - malformed input
+     */
+    let validated = validate(input)?;
+    let trimmed = validated.trim();
+    let result = transform(trimmed);
+    Ok(result)
+}
+RSEOF
+git add lib.rs && git commit -q -m "change"
+
+# --- JavaScript template literal ---
+REPO="$DIR/context-demo-js"
+rm -rf "$REPO"
+mkdir -p "$REPO" && cd "$REPO" && git init -q
+
+cat > render.js << 'JSEOF'
+function render(data) {
+    const html = `
+        <div class="container">
+            <header>
+                <h1>${data.title}</h1>
+                <nav>${data.breadcrumbs}</nav>
+            </header>
+            <main>
+                <p>${data.description}</p>
+                <ul>${data.items}</ul>
+            </main>
+        </div>
+    `;
+    const element = document.createElement("div");
+    element.innerHTML = html;
+    return element;
+}
+JSEOF
+git add render.js && git commit -q -m "initial"
+
+cat > render.js << 'JSEOF'
+function render(data) {
+    const html = `
+        <div class="container">
+            <header>
+                <h1>${data.title}</h1>
+                <nav>${data.breadcrumbs}</nav>
+            </header>
+            <main>
+                <p>${data.description}</p>
+                <ul>${data.items}</ul>
+            </main>
+        </div>
+    `;
+    const wrapper = document.createElement("section");
+    wrapper.innerHTML = html;
+    wrapper.classList.add("rendered");
+    return wrapper;
+}
+JSEOF
+git add render.js && git commit -q -m "change"
+
+echo "Created repos:"
+echo "  $DIR/context-demo-python"
+echo "  $DIR/context-demo-rust"
+echo "  $DIR/context-demo-js"
+echo
+echo "Try:"
+echo "  cd $DIR/context-demo-python"
+echo "  git diff HEAD~1 | delta                    # bug"
+echo "  git diff -U9999 HEAD~1 | delta -U 3        # fix"

src/cli.rs

@@ -184,6 +184,15 @@ pub struct Opt {
     /// doesn't support it, then delta will fall back to `diff` instead of `git diff`.
     pub diff_args: String,
 
+    #[arg(short = 'U', long = "context-lines", value_name = "N")]
+    /// Display at most N context lines around each change.
+    ///
+    /// When the input diff has more context lines than N, delta will use all of them for syntax
+    /// highlighting but only display N. This can fix incorrect syntax highlighting around multiline
+    /// constructs (multiline strings, block comments, etc). Use this in conjunction with git's -U
+    /// option, e.g. `git diff -U9999`.
+    pub context_lines: Option<usize>,
+
     #[arg(long = "diff-highlight")]
     /// Emulate diff-highlight.
     ///

src/config.rs

@@ -46,6 +46,7 @@ pub struct Config {
     pub blame_timestamp_format: String,
     pub blame_timestamp_output_format: Option<String>,
     pub color_only: bool,
+    pub context_lines: Option<usize>,
     pub commit_regex: Regex,
     pub commit_style: Style,
     pub cwd_of_delta_process: Option<PathBuf>,
@@ -295,6 +296,7 @@ impl From<cli::Opt> for Config {
             blame_timestamp_output_format: opt.blame_timestamp_output_format,
             commit_style: styles["commit-style"],
             color_only: opt.color_only,
+            context_lines: opt.context_lines,
             commit_regex,
             cwd_of_delta_process,
             cwd_of_user_shell_process,

src/delta.rs

@@ -186,6 +186,7 @@ impl<'a> StateMachine<'a> {
         self.handle_pending_line_with_diff_name()?;
         self.painter.paint_buffered_minus_and_plus_lines();
         self.painter.emit()?;
+        self.painter.flush_hunk_output()?;
         Ok(())
     }
 

src/git_config/mod.rs

@@ -253,6 +253,20 @@ impl GitConfigGet for usize {
     }
 }
 
+impl GitConfigGet for Option<usize> {
+    fn git_config_get(key: &str, git_config: &GitConfig) -> Option<Self> {
+        if let Some(s) = git_config.config_from_env_var.get(key) {
+            if let Ok(n) = s.parse::<usize>() {
+                return Some(Some(n));
+            }
+        }
+        match git_config.config.get_i64(key) {
+            Ok(value) => Some(Some(value as usize)),
+            _ => None,
+        }
+    }
+}
+
 impl GitConfigGet for f64 {
     fn git_config_get(key: &str, git_config: &GitConfig) -> Option<Self> {
         if let Some(s) = git_config.config_from_env_var.get(key) {

src/handlers/hunk_header.rs

@@ -157,6 +157,7 @@ impl StateMachine<'_> {
         self.painter.paint_buffered_minus_and_plus_lines();
         self.painter.set_highlighter();
         self.painter.emit()?;
+        self.painter.flush_hunk_output()?;
 
         let ParsedHunkHeader {
             code_fragment,

src/options/get.rs

@@ -124,6 +124,7 @@ pub trait GetOptionValue {
 }
 
 impl GetOptionValue for Option<String> {}
+impl GetOptionValue for Option<usize> {}
 impl GetOptionValue for String {}
 impl GetOptionValue for bool {}
 impl GetOptionValue for f64 {}

src/options/option_value.rs

@@ -5,6 +5,7 @@ pub enum OptionValue {
     Boolean(bool),
     Float(f64),
     OptionString(Option<String>),
+    OptionInt(Option<usize>),
     String(String),
     Int(usize),
 }
@@ -81,6 +82,21 @@ impl From<OptionValue> for String {
     }
 }
 
+impl From<Option<usize>> for OptionValue {
+    fn from(value: Option<usize>) -> Self {
+        OptionValue::OptionInt(value)
+    }
+}
+
+impl From<OptionValue> for Option<usize> {
+    fn from(value: OptionValue) -> Self {
+        match value {
+            OptionValue::OptionInt(value) => value,
+            _ => delta_unreachable("Error converting OptionValue to Option<usize>."),
+        }
+    }
+}
+
 impl From<usize> for OptionValue {
     fn from(value: usize) -> Self {
         OptionValue::Int(value)

src/options/set.rs

@@ -139,6 +139,7 @@ pub fn set_options(
             commit_decoration_style,
             commit_regex,
             commit_style,
+            context_lines,
             default_language,
             diff_args,
             diff_stat_align_width,