tdf#173803 Set WM_CLASS with Qt6 on X11

The setApplicationID method is now called for all Qt versions on
X11. For Qt6, xcb_connection_t* is obtained in a new way, by
calling QNativeInterface::QX11Application, instead of the
deprecated X11Helper.

This change is based on these Qt6 docs:
https://doc.qt.io/qt-6/extras-changes-qt6.html#changes-to-qt-x11-extras

Change-Id: I51da53dd7d4cd7e080812cfd0c59d6197cac4a6f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/211643
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/qt5/QtFrame.cxx

@@ -1177,7 +1177,7 @@ void QtFrame::SetScreenNumber(unsigned int nScreen)
 
 void QtFrame::SetApplicationID(const OUString& rWMClass)
 {
-#if CHECK_QT5_USING_X11
+#if CHECK_ANY_QT_USING_X11
     if (GetQtInstance().GetPlatform() != Platform::Xcb || !m_pTopLevel)
         return;
 

vcl/qt5/QtX11Support.cxx

@@ -20,7 +20,16 @@
 
 void QtX11Support::setApplicationID(const xcb_window_t nWinId, std::u16string_view rWMClass)
 {
+    xcb_connection_t* pConnection = nullptr;
 #if CHECK_QT5_USING_X11
+    pConnection = QX11Info::connection();
+#else
+    auto* x11Application = qGuiApp->nativeInterface<QNativeInterface::QX11Application>();
+    if (x11Application)
+        pConnection = x11Application->connection();
+#endif
+    if (!pConnection)
+        return;
     OString aResClass = OUStringToOString(rWMClass, RTL_TEXTENCODING_ASCII_US);
     const char* pResClass
         = !aResClass.isEmpty() ? aResClass.getStr() : X11Helper::getFrameClassName();
@@ -32,13 +41,9 @@ void QtX11Support::setApplicationID(const xcb_window_t nWinId, std::u16string_vi
     memcpy(data, aResName.getStr(), aResName.getLength() + 1);
     memcpy(data + aResName.getLength() + 1, pResClass, strlen(pResClass) + 1);
 
-    xcb_change_property(QX11Info::connection(), XCB_PROP_MODE_REPLACE, nWinId, XCB_ATOM_WM_CLASS,
+    xcb_change_property(pConnection, XCB_PROP_MODE_REPLACE, nWinId, XCB_ATOM_WM_CLASS,
                         XCB_ATOM_STRING, 8, data_len, data);
     delete[] data;
-#else
-    Q_UNUSED(nWinId);
-    Q_UNUSED(rWMClass);
-#endif
 }
 
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