FIx: Allow ext flags to be opt-out by default rather than opt-in

app.go

@@ -113,6 +113,9 @@ type App struct {
 	UseShortOptionHandling bool
 	// Enable suggestions for commands and flags
 	Suggest bool
+	// Allows global flags set by libraries which use flag.XXXVar(...) directly
+	// to be parsed through this library
+	AllowExtFlags bool
 
 	didSetup bool
 
@@ -199,13 +202,15 @@ func (a *App) Setup() {
 		a.ErrWriter = os.Stderr
 	}
 
-	// add global flags added by other packages
-	flag.VisitAll(func(f *flag.Flag) {
-		// skip test flags
-		if !strings.HasPrefix(f.Name, ignoreFlagPrefix) {
-			a.Flags = append(a.Flags, &extFlag{f})
-		}
-	})
+	if a.AllowExtFlags {
+		// add global flags added by other packages
+		flag.VisitAll(func(f *flag.Flag) {
+			// skip test flags
+			if !strings.HasPrefix(f.Name, ignoreFlagPrefix) {
+				a.Flags = append(a.Flags, &extFlag{f})
+			}
+		})
+	}
 
 	var newCommands []*Command