Qt6 port:

* QApplication::setFallbackSessionManagementEnabled has been removed
* QString::medRef() has been removed. Use QString::mid() again.
* QTextStream::setCodec has been removed
* Use operator QVariant of the QFont class to make code Qt5 and Qt6 compatible
* Signature of QTreeWidget::mimeData() has changed in Qt6. Remove TreeWidget::mimeData() because it doesn't change the implementation
* QLayout::setMargin() is deprecated in Qt5 and has been removed in Qt6. Use QLayout::setContentsMargins()
* QDateTime::toTime_t() is deprecated in Qt5 and has been removed in Qt6. Use QDateTime::toSecsSinceEpoch()
* QDesktopWidget is deprecated in Qt5 and has been removed in Qt6. Use QScreen

src/Mod/Sketcher/Gui/ViewProviderSketch.cpp

@@ -39,13 +39,13 @@
 # include <QAction>
 # include <QApplication>
 # include <QColor>
-# include <QDesktopWidget>
 # include <QDialog>
 # include <QFont>
 # include <QKeyEvent>
 # include <QMenu>
 # include <QMessageBox>
 # include <QPainter>
+# include <QScreen>
 # include <QTextStream>
 #endif
 
@@ -3629,7 +3629,7 @@ int ViewProviderSketch::defaultFontSizePixels() const
 }
 
 int ViewProviderSketch::getApplicationLogicalDPIX() const {
-    return QApplication::desktop()->logicalDpiX();
+    return int(QApplication::primaryScreen()->logicalDotsPerInchX());
 }
 
 int ViewProviderSketch::getViewOrientationFactor() const {