From 3d45f23b022878444454ec1a47c25678b88868c6 Mon Sep 17 00:00:00 2001
From: Ilya Fedin <fedin-ilja2010@ya.ru>
Date: Sat, 2 Jul 2022 13:56:00 +0400
Subject: [PATCH] QGtk3Theme: Ensure gtk uses the same windowing system as Qt

Fixes: QTBUG-69354
Pick-to: 6.4 6.3 6.2 5.15
Change-Id: I254c5bf98bc3b2cc2577159e475a2105438bb96b
Reviewed-by: Liang Qi <liang.qi@qt.io>
---
 src/plugins/platformthemes/gtk3/qgtk3theme.cpp | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/src/plugins/platformthemes/gtk3/qgtk3theme.cpp b/src/plugins/platformthemes/gtk3/qgtk3theme.cpp
index 22a079732a7..51e19175d6f 100644
--- a/src/plugins/platformthemes/gtk3/qgtk3theme.cpp
+++ b/src/plugins/platformthemes/gtk3/qgtk3theme.cpp
@@ -6,6 +6,7 @@
 #include "qgtk3menu.h"
 #include <QVariant>
 #include <QtCore/qregularexpression.h>
+#include <QGuiApplication>
 
 #undef signals
 #include <gtk/gtk.h>
@@ -14,6 +15,8 @@
 
 QT_BEGIN_NAMESPACE
 
+using namespace Qt::StringLiterals;
+
 const char *QGtk3Theme::name = "gtk3";
 
 template <typename T>
@@ -49,6 +52,14 @@ void gtkMessageHandler(const gchar *log_domain,
 
 QGtk3Theme::QGtk3Theme()
 {
+    // Ensure gtk uses the same windowing system, but let it
+    // fallback in case GDK_BACKEND environment variable
+    // filters the preferred one out
+    if (QGuiApplication::platformName().startsWith("wayland"_L1))
+        gdk_set_allowed_backends("wayland,x11");
+    else if (QGuiApplication::platformName() == "xcb"_L1)
+        gdk_set_allowed_backends("x11,wayland");
+
     // gtk_init will reset the Xlib error handler, and that causes
     // Qt applications to quit on X errors. Therefore, we need to manually restore it.
     int (*oldErrorHandler)(Display *, XErrorEvent *) = XSetErrorHandler(nullptr);