Always open a text control's context menu on the same screen

It's very disorienting when a context menu pops up on a different
screen from where the right mouse button was clicked.
QWidgetPrivate::setScreenForPoint() can give incorrect results in
multi-screen desktop configurations; but we can give the menu a hint
by setting initialScreenIndex, which was originally used only when
QDesktopScreen or QDesktopScreenWidget is the parent (see
b3fc5e1ea3 ).

Task-number: QTBUG-76162
Pick-to: 5.15
Pick-to: 5.12
Change-Id: Icd4abdc32564430147e6c63a8ea70d483d25cddd
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/widgets/qwidgettextcontrol.cpp

@@ -58,6 +58,7 @@
 #include "private/qapplication_p.h"
 #include "private/qtextdocumentlayout_p.h"
 #include "private/qabstracttextdocumentlayout_p.h"
+#include "private/qmenu_p.h"
 #include "qtextdocument.h"
 #include "private/qtextdocument_p.h"
 #include "qtextlist.h"
@@ -1943,6 +1944,10 @@ void QWidgetTextControlPrivate::contextMenuEvent(const QPoint &screenPos, const
     if (!menu)
         return;
     menu->setAttribute(Qt::WA_DeleteOnClose);
+    if (auto *window = static_cast<QWidget *>(parent)->window()->windowHandle()) {
+        QMenuPrivate::get(menu)->topData()->initialScreenIndex =
+                QGuiApplication::screens().indexOf(window->screen());
+    }
     menu->popup(screenPos);
 #endif
 }