Add terminal width fallback via stty if on Windows/MSYS2

Also new workarounds.rs file, and DELTA_NO_WORKAROUNDS env var
to disable these.

src/options/set.rs

@@ -609,7 +609,8 @@ fn set_widths_and_isatty(opt: &mut cli::Opt) {
 
     // If one extra character for e.g. `less --status-column` is required use "-1"
     // as an argument, also see #41, #10, #115 and #727.
-    opt.computed.available_terminal_width = term_stdout.size().1 as usize;
+    opt.computed.available_terminal_width =
+        crate::utils::workarounds::windows_msys2_width_fix(term_stdout.size(), &term_stdout);
 
     let (decorations_width, background_color_extends_to_terminal_width) = match opt.width.as_deref()
     {

src/utils/mod.rs

@@ -5,3 +5,4 @@ pub mod process;
 pub mod regex_replacement;
 pub mod round_char_boundary;
 pub mod syntect;
+pub mod workarounds;

src/utils/workarounds.rs

@@ -0,0 +1,60 @@
+// env var which should disable workarounds
+const NO_WORKAROUNDS: &str = "DELTA_NO_WORKAROUNDS";
+
+// Work around a bug in the 'console' crate: On Windows it can not determine the width of
+// an MSYS2 / MINGW64 terminal (e.g. from Git-Bash) correctly.
+// Instead use the usually included stty util from the MSYS2 distribution.
+#[cfg(target_os = "windows")]
+pub fn windows_msys2_width_fix(height_width: (u16, u16), term_stdout: &console::Term) -> usize {
+    fn guess_real_width(current_width: u16, term_stdout: &console::Term) -> Option<u16> {
+        use std::process::{Command, Stdio};
+
+        let term_var = std::env::var("TERM").ok()?;
+        // More checks before actually calling stty.
+        if term_var.starts_with("xterm")
+            && term_stdout.is_term()
+            && term_stdout.features().is_msys_tty()
+        {
+            if std::env::var(NO_WORKAROUNDS).is_ok() {
+                return Some(current_width);
+            }
+
+            // stderr/2 is passed to the Command below.
+            let pseudo_term = "/dev/fd/2";
+
+            // Read width via stty helper program (e.g. "C:\Program Files\Git\usr\bin\stty.exe")
+            // which gets both the MSYS2 and cmd.exe width right.
+            let result = Command::new("stty")
+                .stderr(Stdio::inherit())
+                .arg("-F")
+                .arg(pseudo_term)
+                .arg("size")
+                .output()
+                .ok()?;
+
+            if result.status.success() {
+                let size = std::str::from_utf8(&result.stdout).ok()?;
+                let mut it = size.split_whitespace();
+                let _height = it.next()?;
+                return it.next().map(|width| width.parse().ok())?;
+            }
+        }
+        None
+    }
+
+    // Calling an external binary is slow, so make sure this is actually necessary.
+    // The fallback values of 25 lines by 80 columns (sometimes zero indexed) are a good
+    // indicator.
+    let (height, width) = height_width;
+    match (height, width) {
+        (24..=25, 79..=80) => guess_real_width(width, term_stdout).unwrap_or(width),
+        _ => width,
+    }
+    .into()
+}
+
+#[cfg(not(target_os = "windows"))]
+pub fn windows_msys2_width_fix(height_width: (u16, u16), _: &console::Term) -> usize {
+    let _ = NO_WORKAROUNDS;
+    height_width.1.into()
+}