It seems the mac terminal doesn't do "bold off" so it sticks with bold! This is independent of what shell that's used.

An alternative approach that probably is more portable is to switch off all "styles":

diff --git a/src/tool_cb_hdr.c b/src/tool_cb_hdr.c
index 88ce5e13b..d317b9f39 100644
--- a/src/tool_cb_hdr.c
+++ b/src/tool_cb_hdr.c
@@ -40,11 +40,11 @@ static char *parse_filename(const char *ptr, size_t len);
 #ifdef WIN32
 #define BOLD
 #define BOLDOFF
 #else
 #define BOLD "\x1b[1m"
-#define BOLDOFF "\x1b[21m"
+#define BOLDOFF "\x1b[0m"
 #endif
 
 /*
 ** callback for CURLOPT_HEADERFUNCTION
 */

For the record, the more common unbold sequence is \e[22m. That's the one found in Xterm's documentation for example. I don't know where \e[21m came from. Resetting all styles is even more reliable, I guess.

Using guake on Linux (vte) I was getting lots of double-underlined everything, which bled into the prompt after curl was finished: https://paste.xinu.at/MeNlxzN/

(So, not just macOS.)

But this commit fixes it.

@grawity xterm's documentation is actually at http://invisible-island.net/xterm/ctlseqs/ctlseqs.html (and it documents \e[21m too):

CSI Pm m  Character Attributes (SGR).
        Ps = 2 1  -> Doubly-underlined (ECMA-48).

A fix has already landed! We don't need more details on the error but we could use some confirmations of the fix...

But this commit fixes it.