fix: static server in sub app with mount (gofiber#3104)

router.go

@@ -5,6 +5,7 @@
 package fiber
 
 import (
+	"bytes"
 	"fmt"
 	"html"
 	"sort"
@@ -357,6 +358,12 @@ func (app *App) registerStatic(prefix, root string, config ...Static) {
 		IndexNames:           []string{"index.html"},
 		PathRewrite: func(fctx *fasthttp.RequestCtx) []byte {
 			path := fctx.Path()
+			mountPath := app.MountPath()
+			if n := len(mountPath); n > 0 {
+				if bytes.Equal(path[:n], utils.UnsafeBytes(mountPath)) {
+					path = path[n:]
+				}
+			}
 			if len(path) >= prefixLen {
 				if isStar && app.getString(path[0:prefixLen]) == prefix {
 					path = append(path[0:0], '/')

router_test.go

@@ -471,6 +471,37 @@ func Test_Route_Static_HasPrefix(t *testing.T) {
 	body, err = io.ReadAll(resp.Body)
 	utils.AssertEqual(t, nil, err, "app.Test(req)")
 	utils.AssertEqual(t, true, strings.Contains(app.getString(body), "color"))
+
+	app = New()
+	app.Static("/css", dir)
+
+	resp, err = app.Test(httptest.NewRequest(MethodGet, "/css/style.css", nil))
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, 200, resp.StatusCode, "Status code")
+
+	body, err = io.ReadAll(resp.Body)
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, true, strings.Contains(app.getString(body), "color"))
+}
+
+func Test_Route_Static_SubApp(t *testing.T) {
+	t.Parallel()
+
+	dir := "./.github/testdata/fs/css"
+	app := New()
+
+	subApp := New()
+	subApp.Static("/css", dir)
+
+	app.Mount("/sub", subApp)
+
+	resp, err := app.Test(httptest.NewRequest(MethodGet, "/sub/css/style.css", nil))
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, 200, resp.StatusCode, "Status code")
+
+	body, err := io.ReadAll(resp.Body)
+	utils.AssertEqual(t, nil, err, "app.Test(req)")
+	utils.AssertEqual(t, true, strings.Contains(app.getString(body), "color"))
 }
 
 func Test_Router_NotFound(t *testing.T) {