diff --git a/src/libs/utils/filepath.h b/src/libs/utils/filepath.h
index 541a9555f74..484201383f4 100644
--- a/src/libs/utils/filepath.h
+++ b/src/libs/utils/filepath.h
@@ -234,9 +234,10 @@ public:
     [[nodiscard]] static int rootLength(const QStringView path); // Assumes no scheme and host
     [[nodiscard]] static int schemeAndHostLength(const QStringView path);
 
+    static QString calcRelativePath(const QString &absolutePath, const QString &absoluteAnchorPath);
+
 private:
     friend class ::tst_fileutils;
-    static QString calcRelativePath(const QString &absolutePath, const QString &absoluteAnchorPath);
     void setPath(QStringView path);
     void setFromString(const QString &filepath);
     DeviceFileAccess *fileAccess() const;
diff --git a/src/plugins/coreplugin/corejsextensions.cpp b/src/plugins/coreplugin/corejsextensions.cpp
index 68a44dd7076..a7934e2ede4 100644
--- a/src/plugins/coreplugin/corejsextensions.cpp
+++ b/src/plugins/coreplugin/corejsextensions.cpp
@@ -16,8 +16,9 @@
 #include <QVariant>
 #include <QVersionNumber>
 
-namespace Core {
-namespace Internal {
+using namespace Utils;
+
+namespace Core::Internal {
 
 QString UtilsJsExtension::qtVersion() const
 {
@@ -83,7 +84,9 @@ QString UtilsJsExtension::absoluteFilePath(const QString &in) const
 
 QString UtilsJsExtension::relativeFilePath(const QString &path, const QString &base) const
 {
-    return QDir(base).relativeFilePath(path);
+    const FilePath basePath = FilePath::fromString(base).cleanPath();
+    const FilePath filePath = FilePath::fromString(path).cleanPath();
+    return FilePath::calcRelativePath(filePath.toFSPathString(), basePath.toFSPathString());
 }
 
 bool UtilsJsExtension::exists(const QString &in) const
@@ -175,5 +178,4 @@ QString UtilsJsExtension::qtQuickVersion(const QString &filePath) const
     return QLatin1String("2.15");
 }
 
-} // namespace Internal
-} // namespace Core
+} // Core::Internal
diff --git a/src/plugins/coreplugin/corejsextensions.h b/src/plugins/coreplugin/corejsextensions.h
index 4c97aa18ad9..dbb4b94ec9a 100644
--- a/src/plugins/coreplugin/corejsextensions.h
+++ b/src/plugins/coreplugin/corejsextensions.h
@@ -8,9 +8,7 @@
 #include <QObject>
 #include <QSet>
 
-namespace Core {
-
-namespace Internal {
+namespace Core::Internal {
 
 class UtilsJsExtension : public QObject
 {
@@ -59,5 +57,4 @@ public:
     Q_INVOKABLE QString qtQuickVersion(const QString &filePath) const;
 };
 
-} // namespace Internal
-} // namespace Core
+} // Core::Internal