Map toolbar drag delta from native pixels

The delta (drag distance) for dragging a tool button with in a tool
bar was calculated by subtracting the global mouse press position from
the drag event's global position.

This has lead to a miscalculation when dragging the button across
screens with different resolutions. The new relative position within
the tool bar became negative, which eventually has lead to resizing of
other tool buttons in the same tool bar.

This patch calculates the delta based on native pixels, which ensures
a correct value in all cases. It falls back to the existing calculation
if no window handle can be established from the tool bar.

Fixes: QTBUG-103720
Pick-to: 6.5
Change-Id: I09168b597f6c43a119041d00f5b07e1895fdf4b3
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/widgets/widgets/qtoolbar.cpp

@@ -24,6 +24,7 @@
 #include <qtimer.h>
 #include <private/qwidgetaction_p.h>
 #include <private/qmainwindowlayout_p.h>
+#include <private/qhighdpiscaling_p.h>
 
 #ifdef Q_OS_MACOS
 #include <qpa/qplatformnativeinterface.h>
@@ -309,7 +310,12 @@ bool QToolBarPrivate::mouseMoveEvent(QMouseEvent *event)
         const QPoint globalPressPos = q->mapToGlobal(q->isRightToLeft() ? rtl : state->pressPos);
         int pos = 0;
 
-        QPoint delta = event->globalPosition().toPoint() - globalPressPos;
+        const QWindow *handle = q->window() ? q->window()->windowHandle() : nullptr;
+        const QPoint delta = handle
+                ? QHighDpi::fromNativePixels(event->globalPosition(), handle).toPoint()
+                  - QHighDpi::fromNativePixels(globalPressPos, handle)
+                : event->globalPosition().toPoint() - globalPressPos;
+
         if (orientation == Qt::Vertical) {
             pos = q->y() + delta.y();
         } else {