xwayland: Also follow the primary output

We want to update the XRandR information so that X11 apps know which is
the primary display, since some of them make use of it to position
themselves.

src/xwl/xwayland.cpp

@@ -14,9 +14,11 @@
 #include "dnd.h"
 #include "xwldrophandler.h"
 
+#include "abstract_output.h"
 #include "main_wayland.h"
 #include "options.h"
 #include "utils.h"
+#include "platform.h"
 #include "wayland_server.h"
 #include "xcbutils.h"
 #include "xwayland_logging.h"
@@ -215,6 +217,7 @@ void Xwayland::stop()
 
 void Xwayland::stopInternal()
 {
+    disconnect(kwinApp()->platform(), &Platform::primaryOutputChanged, this, &Xwayland::updatePrimary);
     Q_ASSERT(m_xwaylandProcess);
     m_app->setClosingX11Connection(true);
 
@@ -400,9 +403,34 @@ void Xwayland::handleXwaylandReady()
     qputenv("XAUTHORITY", m_xAuthority.toUtf8());
     m_app->setProcessStartupEnvironment(env);
 
+    connect(kwinApp()->platform(), &Platform::primaryOutputChanged, this, &Xwayland::updatePrimary);
+    updatePrimary(kwinApp()->platform()->primaryOutput());
+
     Xcb::sync(); // Trigger possible errors, there's still a chance to abort
 }
 
+void Xwayland::updatePrimary(AbstractOutput *primaryOutput)
+{
+    Xcb::RandR::ScreenResources resources(rootWindow());
+    xcb_randr_crtc_t *crtcs = resources.crtcs();
+    if (!crtcs) {
+        return;
+    }
+
+    for (int i = 0; i < resources->num_crtcs; ++i) {
+        Xcb::RandR::CrtcInfo crtcInfo(crtcs[i], resources->config_timestamp);
+        const QRect geometry = crtcInfo.rect();
+        if (geometry.topLeft() == primaryOutput->geometry().topLeft()) {
+            auto outputs = crtcInfo.outputs();
+            if (outputs && crtcInfo->num_outputs > 0) {
+                qCDebug(KWIN_XWL) << "Setting primary" << primaryOutput << outputs[0];
+                xcb_randr_set_output_primary(kwinApp()->x11Connection(), rootWindow(), outputs[0]);
+                break;
+            }
+        }
+    }
+}
+
 void Xwayland::handleSelectionLostOwnership()
 {
     qCWarning(KWIN_XWL) << "Somebody else claimed ownership of WM_S0. This should never happen!";

src/xwl/xwayland.h

@@ -20,6 +20,7 @@ class KSelectionOwner;
 
 namespace KWin
 {
+class AbstractOutput;
 class ApplicationWaylandAbstract;
 class XwaylandSocket;
 
@@ -115,6 +116,7 @@ private:
     void installSocketNotifier();
     void uninstallSocketNotifier();
     void maybeDestroyReadyNotifier();
+    void updatePrimary(AbstractOutput *primaryOutput);
 
     bool startInternal();
     void stopInternal();