Add QScreen::virtualSiblingAt() and use it in QMenubarPrivate::popupAction

QGuiApplication::screenAt() is documented "If the point maps to more
than one set of virtual siblings, the first match is returned."
But in many cases it's possible to start from a known screen and
consider only its siblings, as when deciding where to open a QMenu
from a QMenuBar: the QMenuBar is already shown on some screen(s),
so the QMenu must be shown on a sibling from that set.  This function
should be useful in other such cases too, hence it might as well
be public API.

Task-number: QTBUG-76162
Change-Id: I83c74b40eb53f56fb285a6074a3dc2c0ea9c570b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qguiapplication.cpp

@@ -1040,7 +1040,9 @@ QList<QScreen *> QGuiApplication::screens()
 
     The \a point is in relation to the virtualGeometry() of each set of virtual
     siblings. If the point maps to more than one set of virtual siblings the first
-    match is returned.
+    match is returned.  If you wish to search only the virtual desktop siblings
+    of a known screen (for example siblings of the screen of your application
+    window \c QWidget::windowHandle()->screen()), use QScreen::virtualSiblingAt().
 
     \since 5.10
 */

src/gui/kernel/qscreen.cpp

@@ -699,6 +699,25 @@ void QScreenPrivate::updatePrimaryOrientation()
     primaryOrientation = geometry.width() >= geometry.height() ? Qt::LandscapeOrientation : Qt::PortraitOrientation;
 }
 
+/*!
+    Returns the screen at \a point within the set of \l QScreen::virtualSiblings(),
+    or \c nullptr if outside of any screen.
+
+    The \a point is in relation to the virtualGeometry() of each set of virtual
+    siblings.
+
+    \since 5.15
+*/
+QScreen *QScreen::virtualSiblingAt(const QPoint &point)
+{
+    const auto &siblings = virtualSiblings();
+    for (QScreen *sibling : siblings) {
+        if (sibling->geometry().contains(point))
+            return sibling;
+    }
+    return nullptr;
+}
+
 /*!
     Creates and returns a pixmap constructed by grabbing the contents
     of the given \a window restricted by QRect(\a x, \a y, \a width,

src/gui/kernel/qscreen.h

@@ -125,6 +125,7 @@ public:
     QRect availableGeometry() const;
 
     QList<QScreen *> virtualSiblings() const;
+    QScreen *virtualSiblingAt(const QPoint &point);
 
     QSize virtualSize() const;
     QRect virtualGeometry() const;

src/widgets/widgets/qmenubar.cpp

@@ -324,15 +324,10 @@ void QMenuBarPrivate::popupAction(QAction *action, bool activateFirst)
         QPoint pos(q->mapToGlobal(QPoint(adjustedActionRect.left(), adjustedActionRect.bottom() + 1)));
         QSize popup_size = activeMenu->sizeHint();
         //we put the popup menu on the screen containing the bottom-center of the action rect
-        QScreen *popupScreen = q->window()->windowHandle()->screen();
-        QPoint bottomMiddlePos = pos + QPoint(adjustedActionRect.width() / 2, 0);
-        const auto &siblings = popupScreen->virtualSiblings();
-        for (QScreen *sibling : siblings) {
-            if (sibling->geometry().contains(bottomMiddlePos)) {
-                popupScreen = sibling;
-                break;
-            }
-        }
+        QScreen *menubarScreen = q->window()->windowHandle()->screen();
+        QScreen *popupScreen = menubarScreen->virtualSiblingAt(pos + QPoint(adjustedActionRect.width() / 2, 0));
+        if (!popupScreen)
+            popupScreen = menubarScreen;
         QRect screenRect = popupScreen->geometry();
         pos = QPoint(qMax(pos.x(), screenRect.x()), qMax(pos.y(), screenRect.y()));
         const bool fitUp = (pos.y() - popup_size.height() >= screenRect.top());