xcb: set primary screen more correctly

For example, when having virtual monitor which includes two real
monitors, the primary information in xcb_randr_monitor_info_t
is normally false, because user can only set it for output.

Kudos to Jiang Wu for his first patch and details of the issue.

Done-with: Jiang Wu <wujiang@kylinos.cn>
Change-Id: I6af443ff69d347a6d86efc9c8ea7a5d18f4c3e24
Reviewed-by: JiDe Zhang <zhangjide@uniontech.com>
Reviewed-by: Jiang Wu <wujiang@kylinos.cn>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit a79e2aafd63071da42212f6d30e64aef878154ab)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/xcb/qxcbscreen.cpp

@@ -679,10 +679,11 @@ void QXcbScreen::setMonitor(xcb_randr_monitor_info_t *monitorInfo, xcb_timestamp
         m_sizeMillimeters = virtualDesktop()->physicalSize();
 
     m_outputName = getName(monitorInfo);
-    if (connection()->primaryScreenNumber() == virtualDesktop()->number() && monitorInfo->primary)
-        m_primary = true;
-    else
     m_primary = false;
+    if (connection()->primaryScreenNumber() == virtualDesktop()->number()) {
+        if (monitorInfo->primary || isPrimaryInXScreen())
+            m_primary = true;
+    }
 
     m_cursor = new QXcbCursor(connection(), this);
 
@@ -701,6 +702,17 @@ QString QXcbScreen::defaultName()
     return name;
 }
 
+bool QXcbScreen::isPrimaryInXScreen()
+{
+    auto primary = Q_XCB_REPLY(xcb_randr_get_output_primary, connection()->xcb_connection(), root());
+    if (!primary)
+        qWarning("failed to get the primary output of the screen");
+
+    const bool isPrimary = primary ? (m_monitor ? m_outputs.contains(primary->output) : m_output == primary->output) : false;
+
+    return isPrimary;
+}
+
 QXcbScreen::~QXcbScreen()
 {
     delete m_cursor;

src/plugins/platforms/xcb/qxcbscreen.h

@@ -161,6 +161,7 @@ public:
     void setCrtc(xcb_randr_crtc_t crtc) { m_crtc = crtc; }
     void setMonitor(xcb_randr_monitor_info_t *monitorInfo, xcb_timestamp_t timestamp = XCB_NONE);
     QString defaultName();
+    bool isPrimaryInXScreen();
 
     void windowShown(QXcbWindow *window);
     QString windowManagerName() const { return m_virtualDesktop->windowManagerName(); }