fix: fix not thread-safe settings singleton.

fix not thread-safe way for fetching settings singleton.
remove qApp from Settings' parent and leak it anyway.

app/settings.cpp

@@ -13,6 +13,14 @@
 #include <QKeySequence>
 #include <QMetaEnum>
 
+Settings *Settings::instance()
+{
+    // SAFETY:
+    // This is a global singleton and it is leaked by design.
+    static Settings* INSTANCE = new Settings();
+    return INSTANCE;
+}
+
 namespace QEnumHelper
 {
     template <typename E>
@@ -34,17 +42,6 @@ namespace QEnumHelper
     }
 }
 
-Settings *Settings::m_settings_instance = nullptr;
-
-Settings *Settings::instance()
-{
-    if (!m_settings_instance) {
-        m_settings_instance = new Settings;
-    }
-
-    return m_settings_instance;
-}
-
 bool Settings::useLightCheckerboard() const
 {
     return m_qsettings->value("use_light_checkerboard", false).toBool();
@@ -156,8 +153,8 @@ QString getApplicationDirPath()
 }
 #endif // defined(FLAG_PORTABLE_MODE_SUPPORT) && defined(Q_OS_WIN)
 
-Settings::Settings()
-    : QObject(qApp)
+Settings::Settings(QObject *parent)
+    : QObject(parent)
 {
     QString configPath;
 

app/settings.h

@@ -51,9 +51,7 @@ public:
     void setHiDpiScaleFactorBehavior(Qt::HighDpiScaleFactorRoundingPolicy hidpi);
 
 private:
-    Settings();
-
-    static Settings *m_settings_instance;
+    Settings(QObject *parent = nullptr);
 
     QSettings *m_qsettings;