test: add integration tests for --diff combined with --plain

tests/integration_tests.rs

@@ -3965,3 +3965,149 @@ fn word_wrap_short_line_no_wrap() {
         .success()
         .stdout("Single Line\n");
 }
+
+#[cfg(unix)]
+#[cfg(feature = "git")]
+fn setup_diff_test_repo() -> tempfile::TempDir {
+    use std::process::Command;
+
+    let dir = tempfile::tempdir().expect("can create temporary directory");
+    let repo = dir.path();
+
+    // Initialize a git repo and commit a file
+    Command::new("git")
+        .args(["init"])
+        .current_dir(repo)
+        .output()
+        .expect("git init");
+
+    Command::new("git")
+        .args(["config", "user.email", "test@test.com"])
+        .current_dir(repo)
+        .output()
+        .expect("git config email");
+
+    Command::new("git")
+        .args(["config", "user.name", "Test"])
+        .current_dir(repo)
+        .output()
+        .expect("git config name");
+
+    std::fs::write(repo.join("test.txt"), "line 1\nline 2\nline 3\n")
+        .expect("can write test file");
+
+    Command::new("git")
+        .args(["add", "test.txt"])
+        .current_dir(repo)
+        .output()
+        .expect("git add");
+
+    Command::new("git")
+        .args(["commit", "-m", "initial"])
+        .current_dir(repo)
+        .output()
+        .expect("git commit");
+
+    // Modify the file so --diff has something to show
+    std::fs::write(repo.join("test.txt"), "line 1\nline 2 modified\nline 3\nline 4 added\n")
+        .expect("can write modified test file");
+
+    dir
+}
+
+#[cfg(unix)]
+#[cfg(feature = "git")]
+#[test]
+fn diff_plain_preserves_change_markers() {
+    let repo = setup_diff_test_repo();
+
+    // With --diff --plain, output should contain the change marker column
+    // but not other decorations like line numbers or grid
+    let output = bat()
+        .current_dir(repo.path())
+        .arg("--diff")
+        .arg("--plain")
+        .arg("--color=never")
+        .arg("--decorations=always")
+        .arg("test.txt")
+        .assert()
+        .success()
+        .get_output()
+        .stdout
+        .clone();
+
+    let stdout = std::str::from_utf8(&output).expect("valid utf-8");
+
+    // The output should contain the modified and added lines
+    assert!(
+        stdout.contains("line 2 modified"),
+        "diff plain output should contain modified line, got: {stdout}"
+    );
+    assert!(
+        stdout.contains("line 4 added"),
+        "diff plain output should contain added line, got: {stdout}"
+    );
+
+    // Should NOT contain line numbers (a decoration that --plain disables)
+    assert!(
+        !stdout.contains("   1"),
+        "diff plain output should not contain line numbers, got: {stdout}"
+    );
+}
+
+#[cfg(unix)]
+#[cfg(feature = "git")]
+#[test]
+fn diff_plain_does_not_show_grid_or_header() {
+    let repo = setup_diff_test_repo();
+
+    let output = bat()
+        .current_dir(repo.path())
+        .arg("--diff")
+        .arg("--plain")
+        .arg("--color=never")
+        .arg("--decorations=always")
+        .arg("--terminal-width=80")
+        .arg("test.txt")
+        .assert()
+        .success()
+        .get_output()
+        .stdout
+        .clone();
+
+    let stdout = std::str::from_utf8(&output).expect("valid utf-8");
+
+    // Grid lines use box-drawing characters
+    assert!(
+        !stdout.contains('─'),
+        "diff plain output should not contain grid lines, got: {stdout}"
+    );
+    assert!(
+        !stdout.contains('│'),
+        "diff plain output should not contain grid separators, got: {stdout}"
+    );
+
+    // Header shows "File: <filename>"
+    assert!(
+        !stdout.contains("File:"),
+        "diff plain output should not contain file header, got: {stdout}"
+    );
+}
+
+#[cfg(unix)]
+#[cfg(feature = "git")]
+#[test]
+fn plain_without_diff_still_works() {
+    let repo = setup_diff_test_repo();
+
+    // --plain without --diff should output file content with no decorations at all
+    bat()
+        .current_dir(repo.path())
+        .arg("--plain")
+        .arg("--color=never")
+        .arg("--decorations=always")
+        .arg("test.txt")
+        .assert()
+        .success()
+        .stdout("line 1\nline 2 modified\nline 3\nline 4 added\n");
+}