godoc: make line numbers unselectable
Insert line numbers via a ::before pseudo-element so you can't select
them when you copy text. See the code comment for more information.
Fixes golang/go#20077.
Change-Id: I24a5b17786a52c8107b4f830e824526ba03bc38d
Reviewed-on: https://go-review.googlesource.com/41418
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/godoc/server.go b/godoc/server.go
index 162e5a6..ffe5997 100644
--- a/godoc/server.go
+++ b/godoc/server.go
@@ -621,7 +621,16 @@
// linkWriter, so we have to add line spans as another pass.
n := 1
for _, line := range bytes.Split(buf.Bytes(), []byte("\n")) {
- fmt.Fprintf(saved, "<span id=\"L%d\" class=\"ln\">%6d\t</span>", n, n)
+ // The line numbers are inserted into the document via a CSS ::before
+ // pseudo-element. This prevents them from being copied when users
+ // highlight and copy text.
+ // ::before is supported in 98% of browsers: https://caniuse.com/#feat=css-gencontent
+ // This is also the trick Github uses to hide line numbers.
+ //
+ // The first tab for the code snippet needs to start in column 9, so
+ // it indents a full 8 spaces, hence the two nbsp's. Otherwise the tab
+ // character only indents about two spaces.
+ fmt.Fprintf(saved, `<span id="L%d" class="ln" data-content="%6d"> </span>`, n, n)
n++
saved.Write(line)
saved.WriteByte('\n')
