Fix terminal width detection

This was previously fixed in
cb900c3463 (see commit message), but
broken in #341.

Instead, use both term_size and terminal_size, to maximise our chances
that we can detect the width. Also comment the code with the relevant
terminal_size issue.

Fixes #346

CHANGELOG.md

@@ -7,6 +7,11 @@ UTF-16-LE. Previously it required files to be UTF-8.
 
 Added support for Makefiles.
 
+### Command Line Interface
+
+Fixed terminal width detection when only stderr is a TTY (e.g. when
+using difftastic with git). This was broken in 0.34.
+
 ## 0.34 (released 27th August 2022)
 
 ### Build

Cargo.lock

@@ -207,6 +207,7 @@ dependencies = [
  "rpds",
  "rustc-hash",
  "strsim",
+ "term_size",
  "terminal_size 0.2.1",
  "tree-sitter",
  "tree_magic_mini",
@@ -601,6 +602,16 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "term_size"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e4129646ca0ed8f45d09b929036bafad5377103edd06e50bf574b353d2b08d9"
+dependencies = [
+ "libc",
+ "winapi",
+]
+
 [[package]]
 name = "termcolor"
 version = "1.1.3"

Cargo.toml

@@ -48,6 +48,7 @@ rayon = "1.5.2"
 tree_magic_mini = "3.0.3"
 bumpalo = "3.9.1"
 unicode-width = "0.1.9"
+term_size = "0.3.2"
 
 [dev-dependencies]
 pretty_assertions = "1.2.1"

src/options.rs

@@ -377,7 +377,22 @@ pub fn parse_args() -> Mode {
 /// Choose the display width: try to autodetect, or fall back to a
 /// sensible default.
 fn detect_display_width() -> usize {
-    terminal_size::terminal_size().map_or(80, |(w, _)| w.0 as usize)
+    // terminal_size is actively maintained, but only considers
+    // stdout. This is a problem inside git, where stderr is a TTY
+    // with a size but stdout is piped to less.
+    //
+    // https://github.com/eminence/terminal-size/issues/23
+    if let Some(width) = terminal_size::terminal_size().map(|(w, _)| w.0 as usize) {
+        return width;
+    }
+
+    // term_size is no longer maintained, but it checks all of stdin,
+    // stdout and stderr, so gives better results in may cases.
+    if let Some(width) = term_size::dimensions().map(|(w, _)| w) {
+        return width;
+    }
+
+    80
 }
 
 pub fn should_use_color(color_output: ColorOutput) -> bool {