This data race requires quite of few conditions to appear.

1) The transport has to have HTTP/2 enabled (in goproxy, it's not by default).
2) The target server must be an HTTP/2 one.
3) The request must have a body.
4) The server must return certain erroneus HTTP status code.
5) The client must ignore Connection: close header from the proxy
   (4ec7933).

Somehow, I managed to hit all these conditions, and got this error:

    panic: runtime error: slice bounds out of range [96:48]

    goroutine 341699 [running]:
    bufio.(*Reader).ReadSlice(0xc000b0c1e0, 0x80?)
        /usr/local/go/src/bufio/bufio.go:347 +0x225
    bufio.(*Reader).ReadLine(0xc000b0c1e0)
        /usr/local/go/src/bufio/bufio.go:401 +0x27
    net/textproto.(*Reader).readLineSlice(0xc0013400f0)
        /usr/local/go/src/net/textproto/reader.go:56 +0x99
    net/textproto.(*Reader).ReadLine(...)
        /usr/local/go/src/net/textproto/reader.go:39
    net/http.readRequest(0xc000847d40?)
        /usr/local/go/src/net/http/request.go:1042 +0xba
    net/http.ReadRequest(0xc000b0c1e0?)
        /usr/local/go/src/net/http/request.go:1025 +0x19
    github.com/elazarl/goproxy.(*ProxyHttpServer).handleHttps.func2()
        /root/go/pkg/mod/github.com/elazarl/goproxy@v0.0.0-20220901064549-fbd10ff4f5a1/https.go:221 +0x3db
    created by github.com/elazarl/goproxy.(*ProxyHttpServer).handleHttps
        /root/go/pkg/mod/github.com/elazarl/goproxy@v0.0.0-20220901064549-fbd10ff4f5a1/https.go:211 +0x611

Despite 5), goproxy should not break from non-compliant HTTP clients,
and "Connection: close" is more like of an accidental fix.

See also golang/go#61596 (comment)