fix(paging): skip pager when stdout is not a terminal

src/main.rs

@@ -150,6 +150,8 @@ pub fn run_app(
     let pager_cfg = (&config).into();
     let paging_mode = if capture_output.is_some() {
         PagingMode::Capture
+    } else if config.paging_mode != PagingMode::Always && !io::stdout().is_terminal() {
+        PagingMode::Never
     } else {
         config.paging_mode
     };
@@ -303,3 +305,4 @@ pub fn run_app(
 
     // `output_type` drop impl runs here
 }
+

tests/empty_stdin.rs

@@ -0,0 +1,58 @@
+use std::io::Write;
+use std::process::{Command, Stdio};
+
+fn delta_bin() -> String {
+    std::env::var("CARGO_BIN_EXE_delta").unwrap_or_else(|_| {
+        // Fallback for when not run via cargo test
+        format!("{}/target/debug/delta", env!("CARGO_MANIFEST_DIR"))
+    })
+}
+
+#[test]
+fn test_no_output_on_empty_stdin_when_stdout_piped() {
+    let mut child = Command::new(delta_bin())
+        .args(["--no-gitconfig"])
+        .stdin(Stdio::piped())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::null())
+        .spawn()
+        .expect("failed to spawn delta");
+
+    child.stdin.take().unwrap().write_all(b"").unwrap();
+    let output = child.wait_with_output().unwrap();
+
+    assert!(
+        output.status.success(),
+        "delta exited with error on empty stdin: {}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+    assert_eq!(
+        output.stdout.len(),
+        0,
+        "Expected 0 bytes on empty stdin piped to delta, got {} bytes: {:?}",
+        output.stdout.len(),
+        String::from_utf8_lossy(&output.stdout)
+    );
+}
+
+#[test]
+fn test_empty_stdin_does_not_hang_with_paging_always() {
+    let mut child = Command::new(delta_bin())
+        .args(["--no-gitconfig", "--paging=always", "--pager=cat"])
+        .stdin(Stdio::piped())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::null())
+        .spawn()
+        .expect("failed to spawn delta");
+
+    child.stdin.take().unwrap().write_all(b"").unwrap();
+    let output = child.wait_with_output().unwrap();
+
+    // --paging=always still starts the pager even when stdout is piped.
+    // Using --pager=cat avoids hanging on environments where less waits for a TTY.
+    assert!(
+        output.status.success(),
+        "delta --paging=always exited with error on empty stdin: {}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+}