From 01c739425470990efd607fdf57c9b24033c71300 Mon Sep 17 00:00:00 2001
From: Mike Hommey <mh+mozilla@glandium.org>
Date: Wed, 23 Dec 2015 12:11:45 +0900
Subject: [PATCH] Bug 1234026 - Pass a --display option to gtk_init in content
 processes

---
 dom/ipc/ContentChild.cpp | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/dom/ipc/ContentChild.cpp b/dom/ipc/ContentChild.cpp
index 14a7302..cfec4b7 100644
--- a/dom/ipc/ContentChild.cpp
+++ b/dom/ipc/ContentChild.cpp
@@ -615,17 +615,38 @@ NS_INTERFACE_MAP_BEGIN(ContentChild)
 NS_INTERFACE_MAP_END
 
 bool
 ContentChild::Init(MessageLoop* aIOLoop,
                    base::ProcessId aParentPid,
                    IPC::Channel* aChannel)
 {
 #ifdef MOZ_WIDGET_GTK
-    gtk_init(nullptr, nullptr);
+    // We need to pass a display down to gtk_init because it's not going to
+    // use the one from the environment on its own when deciding which backend
+    // to use, and when starting under XWayland, it may choose to start with
+    // the wayland backend instead of the x11 backend.
+    // We could use gdk_display_open, and gdk_display_manager_set_default_display
+    // but then we'd have to hold onto it and gdk_display_close it at the
+    // right moment, so it's simpler to just pass down a fake argv list.
+    // The DISPLAY environment variable is normally set by the parent process.
+    const char *display_name = PR_GetEnv("DISPLAY");
+    if (display_name) {
+        int argc = 3;
+        const char *argv[] = {
+            nullptr,
+            "--display",
+            display_name,
+            nullptr
+        };
+        char** argv_ = const_cast<char**>(argv);
+        gtk_init(&argc, &argv_);
+    } else {
+        gtk_init(nullptr, nullptr);
+    }
 #endif
 
 #ifdef MOZ_WIDGET_QT
     // sigh, seriously
     nsQAppInstance::AddRef();
 #endif
 
 #ifdef MOZ_X11
-- 
2.6.1