term: fix representation of unmodified F5+

We were unconditionally adding the encoded form of the modifier
mask (eg: appending `;1~` to the sequence) and not all apps know
how to interpret that.

refs: https://github.com/wez/wezterm/issues/227

docs/changelog.markdown

@@ -39,6 +39,8 @@ brief notes about them may accumulate here.
 * Improved terminal emulation conformance; added left/right margin support
   and now passes [esctest](https://gitlab.freedesktop.org/terminal-wg/esctest)
   to a similar degree as iTerm2
+* Fixed an issue where unmodified F5+ would use the CSI-u encoded-modifiers
+  format, and confused eg: `htop`.
 
 ### 20200607-144723-74889cd4
 

term/src/terminalstate.rs

@@ -905,7 +905,14 @@ impl TerminalState {
                         12 => "\x1b[24",
                         _ => bail!("unhandled fkey number {}", n),
                     };
-                    write!(buf, "{};{}~", intro, 1 + encode_modifiers(mods))?;
+                    let encoded_mods = encode_modifiers(mods);
+                    if encoded_mods == 0 {
+                        // If no modifiers are held, don't send the modifier
+                        // sequence, as the modifier encoding is a CSI-u extension.
+                        write!(buf, "{}~", intro)?;
+                    } else {
+                        write!(buf, "{};{}~", intro, 1 + encoded_mods)?;
+                    }
                     buf.as_str()
                 }
             }