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/SketcherSettings.cpp

@@ -51,7 +51,7 @@ SketcherSettings::SketcherSettings(QWidget* parent)
     ui->setupUi(this);
     QGridLayout* gridLayout = new QGridLayout(ui->placeholder);
     gridLayout->setSpacing(0);
-    gridLayout->setMargin(0);
+    gridLayout->setContentsMargins(0, 0, 0, 0);
     form = new SketcherGeneralWidget(ui->placeholder);
     gridLayout->addWidget(form, 0, 0, 1, 1);
 }

src/Mod/Sketcher/Gui/SoDatumLabel.cpp

@@ -36,7 +36,6 @@
 # include <cfloat>
 # include <algorithm>
 # include <QFontMetrics>
-# include <QGLWidget>
 # include <QPainter>
 # include <QPen>
 # include <Inventor/SoPrimitiveVertex.h>
@@ -113,7 +112,7 @@ void SoDatumLabel::drawImage()
         return;
     }
 
-    QFont font(QString::fromLatin1(name.getValue()), size.getValue());
+    QFont font(QString::fromLatin1(name.getValue(), -1), size.getValue());
     QFontMetrics fm(font);
     QString str = QString::fromUtf8(s[0].getString());
 

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 {