QDesktopWidget::screenNumber(QPoint): fix handling of virtual desktops

On X11, QXcbVirtualDesktop represents an X11 screen while QScreen
represents an X11 output.  In the case that there are multiple screens
(possibly with multiple outputs), calculate the screen number correctly:
Find the screen index on the primary virtual desktop first to avoid
obtaining a screen index which doesn't belong to the primary virtual
desktop when screen geometry is similar.

Change-Id: I4cbb29b7aa7cd2125759ffbbbe3db4e934feaeae
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>

1 files changed, 19 insertions, 6 deletions

diff --git a/src/widgets/kernel/qdesktopwidget.cpp b/src/widgets/kernel/qdesktopwidget.cpp
index e586be206e..a1c2aebbe6 100644
--- a/src/widgets/kernel/qdesktopwidget.cpp
+++ b/src/widgets/kernel/qdesktopwidget.cpp
@@ -239,12 +239,25 @@ int QDesktopWidget::screenNumber(const QWidget *w) const
 
 int QDesktopWidget::screenNumber(const QPoint &p) const
 {
-    QList<QScreen *> screens = QGuiApplication::screens();
-
-    for (int i = 0; i < screens.size(); ++i)
-        if (screens.at(i)->geometry().contains(p))
-            return i;
-
+    const QList<QScreen *> screens = QGuiApplication::screens();
+    if (!screens.isEmpty()) {
+        const QList<QScreen *> primaryScreens = screens.first()->virtualSiblings();
+        // Find the screen index on the primary virtual desktop first
+        foreach (QScreen *screen, primaryScreens) {
+            if (screen->geometry().contains(p))
+                return screens.indexOf(screen);
+        }
+        // If the screen index is not found on primary virtual desktop, find
+        // the screen index on all screens except the first which was for
+        // sure in the previous loop. Some other screens may repeat. Find
+        // only when there is more than one virtual desktop.
+        if (screens.count() != primaryScreens.count()) {
+            for (int i = 1; i < screens.size(); ++i) {
+                if (screens[i]->geometry().contains(p))
+                    return i;
+            }
+        }
+    }
     return primaryScreen(); //even better would be closest screen
 }