Fix QToolButton menus showing on primary screens in multiscreen setups

Calculate an initial position based on the current size hint
and pass it to QMenuPrivate::exec(), which does screen checks
based on it.

Amends a78d66743171557d79b16c08be775e3ac15bb4ef.

Pick-to: 5.15
Fixes: QTBUG-84462
Task-number: QTBUG-78966
Change-Id: Icae8d2bc0fb50c4c853cfebaa2b2250fc06542e3
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/widgets/widgets/qtoolbutton.cpp

@@ -805,7 +805,8 @@ void QToolButtonPrivate::popupTimerDone()
     // QTBUG-78966, Delay positioning until after aboutToShow().
     auto positionFunction = [q, horizontal](const QSize &sizeHint) {
         return positionMenu(q, horizontal, sizeHint); };
-    actualMenu->d_func()->exec({}, nullptr, positionFunction);
+    const auto initialPos = positionFunction(actualMenu->sizeHint());
+    actualMenu->d_func()->exec(initialPos, nullptr, positionFunction);
 
     if (!that)
         return;