Improve desktop environment detection.

UNKNOWN now means generic unix theme, rather than Gtk: there
are other KDE/Qt-based desktops, and there's also the case of
no desktop at all (e.g. bare Xvfb, as used by automated builds).

To make this correct for GTK-based desktops, this commits improves
the detection of the current desktop environment.

Change-Id: Ib696624de39d5024527880df7adc26c65b838d15
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/gui/kernel/qplatformservices.cpp

@@ -74,7 +74,10 @@ bool QPlatformServices::openDocument(const QUrl &url)
 /*!
  * \brief QPlatformServices::desktopEnvironment returns the active desktop environment.
  *
- * On Unix this function returns KDE, GNOME or UNKNOWN.
+ * On Unix this function returns the uppercase desktop environment name, such as
+ * KDE, GNOME, UNITY, XFCE, LXDE etc. or UNKNOWN if none was detected.
+ * The primary way to detect the desktop environment is the environment variable
+ * XDG_CURRENT_DESKTOP.
  */
 QByteArray QPlatformServices::desktopEnvironment() const
 {

src/platformsupport/services/genericunix/qgenericunixservices.cpp

@@ -54,16 +54,24 @@ enum { debug = 0 };
 
 static inline QByteArray detectDesktopEnvironment()
 {
-    if (!qEnvironmentVariableIsEmpty("KDE_FULL_SESSION"))
-        return QByteArray("KDE");
-    // Check Unity first, whose older versions also have "GNOME_DESKTOP_SESSION_ID" set.
     const QByteArray xdgCurrentDesktop = qgetenv("XDG_CURRENT_DESKTOP");
-    if (xdgCurrentDesktop == "Unity")
-        return QByteArrayLiteral("UNITY");
-    // GNOME_DESKTOP_SESSION_ID is deprecated for some reason, but still check it
-    if (qgetenv("DESKTOP_SESSION") == "gnome" || !qEnvironmentVariableIsEmpty("GNOME_DESKTOP_SESSION_ID"))
-        return QByteArray("GNOME");
-    return QByteArray("UNKNOWN");
+    if (!xdgCurrentDesktop.isEmpty())
+        return xdgCurrentDesktop.toUpper(); // KDE, GNOME, UNITY, LXDE, MATE, XFCE...
+
+    // Classic fallbacks
+    if (!qEnvironmentVariableIsEmpty("KDE_FULL_SESSION"))
+        return QByteArrayLiteral("KDE");
+    if (!qEnvironmentVariableIsEmpty("GNOME_DESKTOP_SESSION_ID"))
+        return QByteArrayLiteral("GNOME");
+
+    // Fallback to checking $DESKTOP_SESSION (unreliable)
+    const QByteArray desktopSession = qgetenv("DESKTOP_SESSION");
+    if (desktopSession == "gnome")
+        return QByteArrayLiteral("GNOME");
+    if (desktopSession == "xfce")
+        return QByteArrayLiteral("XFCE");
+
+    return QByteArrayLiteral("UNKNOWN");
 }
 
 static inline bool checkExecutable(const QString &candidate, QString *result)

src/platformsupport/themes/genericunix/qgenericunixthemes.cpp

@@ -425,7 +425,11 @@ QStringList QGenericUnixTheme::themeNames()
 #ifndef QT_NO_SETTINGS
             result.push_back(QLatin1String(QKdeTheme::name));
 #endif
-        } else { // Gnome, Unity, other Gtk-based desktops like XFCE.
+        } else if (desktopEnvironment == QByteArrayLiteral("GNOME") ||
+                desktopEnvironment == QByteArrayLiteral("UNITY") ||
+                desktopEnvironment == QByteArrayLiteral("MATE") ||
+                desktopEnvironment == QByteArrayLiteral("XFCE") ||
+                desktopEnvironment == QByteArrayLiteral("LXDE")) { // Gtk-based desktops
             // prefer the GTK2 theme implementation with native dialogs etc.
             result.push_back(QStringLiteral("gtk2"));
             // fallback to the generic Gnome theme if loading the GTK2 theme fails