view: why is the diff line number incremented by one?

[Kyle Meyer <kyle@kyleam.com>]

Diff links position the line one beyond what I expect.  Here's a hunk at
<https://public-inbox.org/meta/20200509082738.23602-2-e@yhbt.net/>:

diff --git a/lib/PublicInbox/Eml.pm b/lib/PublicInbox/Eml.pm
index 4508bd84..80e7c1af 100644
--- a/lib/PublicInbox/Eml.pm
+++ b/lib/PublicInbox/Eml.pm
@@ -71,11 +71,11 @@ sub re_memo ($) {
 # compatible with our uses of Email::MIME
 sub new {
 	my $ref = ref($_[1]) ? $_[1] : \(my $cpy = $_[1]);
-	if ($$ref =~ /(?:\r?\n(\r?\n))/gs) { # likely
+	if ($$ref =~ /\r?\n(\r?\n)/s) { # likely
 		# This can modify $$ref in-place and to avoid memcpy/memmove
 		# on a potentially large $$ref.  It does need to make a
 		# copy for $hdr, though.  Idea stolen from Email::Simple
-		my $hdr = substr($$ref, 0, pos($$ref), ''); # sv_chop on $$ref
+		my $hdr = substr($$ref, 0, $+[0], ''); # sv_chop on $$ref
 		substr($hdr, -(length($1))) = ''; # lower SvCUR
 		bless { hdr => \$hdr, crlf => $1, bdy => $ref }, __PACKAGE__;
 	} elsif ($$ref =~ /^[a-z0-9-]+[ \t]*:/ims && $$ref =~ /(\r?\n)\z/s) {


The link at "-71,11" is
<https://public-inbox.org/meta/4508bd84/s/?b=lib/PublicInbox/Eml.pm#n72>.
When I follow it, I'm taken to line 72, one line below the first context
line above.  I haven't been able to come up with a reason why +1 would
be preferable here, and I didn't spot any explanation when looking
around the code.  It looks like it'd just be a matter of making the
two-line change below, but perhaps that causes breakage that I didn't
notice with my light testing.

diff --git a/lib/PublicInbox/ViewDiff.pm b/lib/PublicInbox/ViewDiff.pm
index 6fe9a0d7..536bb9e3 100644
--- a/lib/PublicInbox/ViewDiff.pm
+++ b/lib/PublicInbox/ViewDiff.pm
@@ -50,12 +50,12 @@ sub diff_hunk ($$$$) {
 
 	if (defined($spfx) && defined($oid_a) && defined($oid_b)) {
 		my ($n) = ($ca =~ /^-([0-9]+)/);
-		$n = defined($n) ? do { ++$n; "#n$n" } : '';
+		$n = defined($n) ? "#n$n" : '';
 
 		$$dst .= qq(@@ <a\nhref="$spfx$oid_a/s/$dctx->{Q}$n">$ca</a>);
 
 		($n) = ($cb =~ /^\+([0-9]+)/);
-		$n = defined($n) ? do { ++$n; "#n$n" } : '';
+		$n = defined($n) ? "#n$n" : '';
 		$$dst .= qq( <a\nhref="$spfx$oid_b/s/$dctx->{Q}$n">$cb</a> @@);
 	} else {
 		$$dst .= "@@ $ca $cb @@";