Fix QDockWidget move between screens with different dpr

When partly moved back and forth between screens
with different dprs (device pixel ratios)
unexpected jumps and size changes could occur.
(See the linked issue for details)

This patch maps global coordinates to native ones
and vice versa (in QDockWidgetPrivate::mouseMoveEvent()),
so that the calculated position is the right coordinate
on the right screen.

Fixes: QTBUG-104205
Change-Id: I0e59792a946e0444fed2e2b857f2f8b140afc9b7
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
(cherry picked from commit 54f328f0e8205480749a6d8d2ebe0e58cb1cdb67)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/widgets/qdockwidget.cpp

@@ -20,6 +20,7 @@
 #include <private/qstylesheetstyle_p.h>
 #include <qpa/qplatformtheme.h>
 
+#include <private/qhighdpiscaling_p.h>
 #include "qdockwidget_p.h"
 #include "qmainwindowlayout_p.h"
 
@@ -752,6 +753,8 @@ void QDockWidgetPrivate::initDrag(const QPoint &pos, bool nca)
 
     state = new QDockWidgetPrivate::DragState;
     state->pressPos = pos;
+    state->globalPressPos = q->mapToGlobal(pos);
+    state->widgetInitialPos = q->pos();
     state->dragging = false;
     state->widgetItem = nullptr;
     state->ownWidgetItem = false;
@@ -981,7 +984,31 @@ bool QDockWidgetPrivate::mouseMoveEvent(QMouseEvent *event)
     if (state && state->dragging && !state->nca) {
         QMargins windowMargins = q->window()->windowHandle()->frameMargins();
         QPoint windowMarginOffset = QPoint(windowMargins.left(), windowMargins.top());
-        QPoint pos = event->globalPosition().toPoint() - state->pressPos - windowMarginOffset;
+
+        // TODO maybe use QScreen API (if/when available) to simplify the below code.
+        const QScreen *orgWdgScreen = QGuiApplication::screenAt(state->widgetInitialPos);
+        const QScreen *screenFrom = QGuiApplication::screenAt(state->globalPressPos);
+        const QScreen *screenTo = QGuiApplication::screenAt(event->globalPosition().toPoint());
+        const QScreen *wdgScreen = q->screen();
+
+        QPoint pos;
+        if (Q_LIKELY(screenFrom && screenTo && wdgScreen && orgWdgScreen)) {
+            const QPoint nativeWdgOrgPos = QHighDpiScaling::mapPositionToNative(
+                    state->widgetInitialPos, orgWdgScreen->handle());
+            const QPoint nativeTo = QHighDpiScaling::mapPositionToNative(
+                    event->globalPosition().toPoint(), screenTo->handle());
+            const QPoint nativeFrom = QHighDpiScaling::mapPositionToNative(state->globalPressPos,
+                                                                           screenFrom->handle());
+
+            // Calculate new nativePos based on startPos + mouse delta move.
+            const QPoint nativeNewPos = nativeWdgOrgPos + (nativeTo - nativeFrom);
+
+            pos = QHighDpiScaling::mapPositionFromNative(nativeNewPos, wdgScreen->handle())
+                    - windowMarginOffset;
+        } else {
+            // Fallback in the unlikely case that source and target screens could not be established
+            pos = event->globalPosition().toPoint() - state->pressPos - windowMarginOffset;
+        }
 
         QDockWidgetGroupWindow *floatingTab = qobject_cast<QDockWidgetGroupWindow*>(parent);
         if (floatingTab && !q->isFloating())

src/widgets/widgets/qdockwidget_p.h

@@ -41,6 +41,8 @@ class QDockWidgetPrivate : public QWidgetPrivate
 
     struct DragState {
         QPoint pressPos;
+        QPoint globalPressPos;
+        QPoint widgetInitialPos;
         bool dragging;
         QLayoutItem *widgetItem;
         bool ownWidgetItem;