feat(help): allow for limiting detected terminal width

Useful when, e.g., the terminal is fullscreen

Closes #653

src/app/help.rs

@@ -94,7 +94,8 @@ impl<'a> Help<'a> {
                hide_pv: bool,
                color: bool,
                cizer: Colorizer,
-               term_w: Option<usize>)
+               term_w: Option<usize>,
+               max_w: Option<usize>)
                -> Self {
         debugln!("fn=Help::new;");
         Help {
@@ -103,7 +104,10 @@ impl<'a> Help<'a> {
             hide_pv: hide_pv,
             term_w: match term_w {
                 Some(width) => if width == 0 { usize::MAX } else { width },
-                None => term_size::dimensions().map_or(120, |(w, _)| w),
+                None => cmp::min(term_size::dimensions().map_or(120, |(w, _)| w), match max_w {
+                    None | Some(0) => usize::MAX,
+                    Some(mw) => mw,
+                }),
             },
             color: color,
             cizer: cizer,
@@ -142,7 +146,7 @@ impl<'a> Help<'a> {
             use_stderr: stderr,
             when: parser.color(),
         };
-        Self::new(w, nlh, hide_v, color, cizer, parser.meta.term_w).write_help(parser)
+        Self::new(w, nlh, hide_v, color, cizer, parser.meta.term_w, parser.meta.max_w).write_help(parser)
     }
 
     /// Writes the parser help to the wrapped stream.

src/app/meta.rs

@@ -14,6 +14,7 @@ pub struct AppMeta<'b> {
     pub help_str: Option<&'b str>,
     pub disp_ord: usize,
     pub term_w: Option<usize>,
+    pub max_w: Option<usize>,
     pub template: Option<&'b str>,
 }
 
@@ -34,6 +35,7 @@ impl<'b> Default for AppMeta<'b> {
             template: None,
             aliases: None,
             term_w: None,
+            max_w: None,
         }
     }
 }
@@ -64,6 +66,7 @@ impl<'b> Clone for AppMeta<'b> {
             template: self.template,
             aliases: self.aliases.clone(),
             term_w: self.term_w,
+            max_w: self.max_w,
         }
     }
 }

src/app/mod.rs

@@ -580,6 +580,34 @@ impl<'a, 'b> App<'a, 'b> {
         self
     }
 
+    /// Sets the max terminal width at which to wrap help messages. Using `0` will ignore terminal
+    /// widths and use source formatting.
+    ///
+    /// `clap` automatically tries to determine the terminal width on Unix, Linux, OSX and Windows
+    /// if the `wrap_help` cargo "feature" has been used while compiling, but one might want to
+    /// limit the size (e.g. when the terminal is running fullscreen).
+    ///
+    /// **NOTE:** This setting applies globally and *not* on a per-command basis.
+    ///
+    /// **NOTE:** This setting must be set **before** any subcommands are added!
+    ///
+    /// # Platform Specific
+    ///
+    /// Only Unix, Linux, OSX and Windows support automatic determination of terminal width.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// # use clap::App;
+    /// App::new("myprog")
+    ///     .max_term_width(100)
+    /// # ;
+    /// ```
+    pub fn max_term_width(mut self, w: usize) -> Self {
+        self.p.meta.max_w = Some(w);
+        self
+    }
+
     /// Adds an [argument] to the list of valid possibilties.
     ///
     /// # Examples