Split separator in StringSliceFlag ignored when using environment variable as source

[cboitel]

My urfave/cli version is

3.6.x (works in 3.5.0)

Checklist

• Are you running the latest v3 release? 3.6.2 is impacted
• Did you check the manual for your release? yes
• Did you perform a search about this problem?

Dependency Management

• My project is using go modules.
• My project is downloading the latest version when go get -u is ran

Describe the bug

We have:

1. a command with SliceFlagSeparator set to space (" ") instead of the default comma
2. a stringsliceflag named "a" (for example) in that command that uses some environment variable (A) as alternate source

Working as expected, meaning slice returned by cmd.StringSlice is [ "0", "1" ]:

• using --a 0 --a 1 command-line options
• using --a "0 1 " command-line

No longer working since 3.6.0, meaning slice returned by cmd.StringSlice is [ "0 1" ]:

• using environment variable A="0 1"

Note that A="0,1" environment variable returns [ "0", "1" ] although this is unexpected since slice separator was set to a spec.

To reproduce

Build a main.go file with the following content and run passing command-line options or environment variable

package main

import (
	"context"
	"fmt"
	"log"
	"os"

	"github.com/urfave/cli/v3"
)

func main() {
	cmd := &cli.Command{
		Flags: []cli.Flag{
			&cli.StringSliceFlag{
				Name:    "a",
				Sources: cli.EnvVars("A"),
			},
		},
		// Ignored by Sources: will use , instead for environment variables
		SliceFlagSeparator: " ",
		Action: func(ctx context.Context, cmd *cli.Command) error {
			for i, v := range cmd.StringSlice("a") {
				fmt.Printf("#%02d: %+v\n", i, v)
			}
			return nil
		},
	}

	if err := cmd.Run(context.Background(), os.Args); err != nil {
		log.Fatal(err)
	}
}

Observed behavior

go run main.go --a 0 --a 1
#00: 0
#01: 1

go run main.go --a "0 1"
#00: 0
#01: 1

A="0 1" go run main.go
#00: 0 1

A="0,1" go run main.go
#00: 0
#01: 1

Expected behavior

go run main.go --a 0 --a 1
#00: 0
#01: 1

go run main.go --a "0 1"
#00: 0
#01: 1

A="0 1" go run main.go
#00: 0
#01: 1

A="0,1" go run main.go
#00: 0,1

Additional context

N/A

Run go version and paste its output here

go version go1.25.6 linux/amd64

Run go env and paste its output here

AR='ar'
CC='cc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='0'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='c++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/xxx/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/xxx/.config/go/env'
GOEXE=''
GOEXPERIMENT='jsonv2'
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1788838360=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/xxx/go.mod'
GOMODCACHE='/home/xxx/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/xxx/go'
GOPRIVATE=''
GOROOT='/usr/lib/go'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/xxx/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.25.6'
GOWORK=''

[cboitel]

I guess MapStringFlag are also impacted but haven't tested it