QgsLayoutUnitComboBox can be linked to spin boxes so that their

values are automatically updated when the combo box unit changes

This means that you can flip between units and things like
the existing width and height are converted immediately to the
new unit

python/gui/layout/qgslayoutnewitempropertiesdialog.sip

@@ -69,6 +69,13 @@ class QgsLayoutItemPropertiesDialog : QDialog
 .. seealso:: referencePoint()
 %End
 
+    void setLayout( QgsLayout *layout );
+%Docstring
+ Sets the ``layout`` associated with the dialog. This allows the dialog
+ to retrieve properties from the layout and perform tasks like automatic
+ conversion of units.
+%End
+
 };
 
 /************************************************************************

python/gui/layout/qgslayoutunitscombobox.sip

@@ -7,6 +7,7 @@
  ************************************************************************/
 
 
+
 class QgsLayoutUnitsComboBox : QComboBox
 {
 %Docstring
@@ -36,6 +37,33 @@ class QgsLayoutUnitsComboBox : QComboBox
 %Docstring
  Sets the ``unit`` currently selected in the combo box.
 .. seealso:: unit()
+%End
+
+    void linkToWidget( QDoubleSpinBox *widget );
+%Docstring
+ Registers a spin box ``widget`` as linked with the combo box.
+
+ Registered spin boxes will automatically be upodated whenever the unit is changed. I.e. a
+ spin box with a value of 100 will be set to 1 when the unit is changed from centimeters to meters.
+
+ A measurement converter() must be set in order for the automatic unit conversion to occur.
+
+.. seealso:: setConverter()
+%End
+
+    QgsLayoutMeasurementConverter *converter() const;
+%Docstring
+ Returns the converter used when automatically converting units for linked widgets.
+.. seealso:: setConverter()
+ :rtype: QgsLayoutMeasurementConverter
+%End
+
+    void setConverter( QgsLayoutMeasurementConverter *converter );
+%Docstring
+ Sets a ``converter`` to use when automatically converting units for linked widgets.
+ The ownership of ``converter`` is not transferred, and converter must exist for the
+ life of the combo box.
+.. seealso:: converter()
 %End
 
   signals:

src/gui/layout/qgslayoutnewitempropertiesdialog.cpp

@@ -15,6 +15,8 @@
 
 #include "qgslayoutnewitempropertiesdialog.h"
 #include "qgssettings.h"
+#include "qgslayout.h"
+
 
 QgsLayoutItemPropertiesDialog::QgsLayoutItemPropertiesDialog( QWidget *parent, Qt::WindowFlags flags )
   : QDialog( parent, flags )
@@ -39,6 +41,11 @@ QgsLayoutItemPropertiesDialog::QgsLayoutItemPropertiesDialog( QWidget *parent, Q
   double lastHeight = settings.value( QStringLiteral( "LayoutDesigner/lastItemHeight" ), QStringLiteral( "50" ) ).toDouble();
   QgsUnitTypes::LayoutUnit lastSizeUnit = static_cast< QgsUnitTypes::LayoutUnit >( settings.value( QStringLiteral( "LayoutDesigner/lastSizeUnit" ) ).toInt() );
   setItemSize( QgsLayoutSize( lastWidth, lastHeight, lastSizeUnit ) );
+
+  mPosUnitsComboBox->linkToWidget( mXPosSpin );
+  mPosUnitsComboBox->linkToWidget( mYPosSpin );
+  mSizeUnitsComboBox->linkToWidget( mWidthSpin );
+  mSizeUnitsComboBox->linkToWidget( mHeightSpin );
 }
 
 void QgsLayoutItemPropertiesDialog::setItemPosition( QgsLayoutPoint position )
@@ -147,3 +154,9 @@ void QgsLayoutItemPropertiesDialog::setReferencePoint( QgsLayoutItem::ReferenceP
       break;
   }
 }
+
+void QgsLayoutItemPropertiesDialog::setLayout( QgsLayout *layout )
+{
+  mSizeUnitsComboBox->setConverter( &layout->context().measurementConverter() );
+  mPosUnitsComboBox->setConverter( &layout->context().measurementConverter() );
+}

src/gui/layout/qgslayoutnewitempropertiesdialog.h

@@ -80,6 +80,13 @@ class GUI_EXPORT QgsLayoutItemPropertiesDialog : public QDialog, private Ui::Qgs
      */
     void setReferencePoint( QgsLayoutItem::ReferencePoint point );
 
+    /**
+     * Sets the \a layout associated with the dialog. This allows the dialog
+     * to retrieve properties from the layout and perform tasks like automatic
+     * conversion of units.
+     */
+    void setLayout( QgsLayout *layout );
+
 };
 
 #endif // QGSLAYOUTNEWITEMPROPERTIESDIALOG_H

src/gui/layout/qgslayoutunitscombobox.cpp

@@ -14,6 +14,7 @@
  ***************************************************************************/
 
 #include "qgslayoutunitscombobox.h"
+#include "qgslayoutmeasurementconverter.h"
 
 QgsLayoutUnitsComboBox::QgsLayoutUnitsComboBox( QWidget *parent )
   : QComboBox( parent )
@@ -34,10 +35,7 @@ QgsLayoutUnitsComboBox::QgsLayoutUnitsComboBox( QWidget *parent )
   setItemData( 6, tr( "Picas" ), Qt::ToolTipRole );
   addItem( tr( "px" ), QgsUnitTypes::LayoutPixels );
   setItemData( 7, tr( "Pixels" ), Qt::ToolTipRole );
-  connect( this, static_cast<void ( QgsLayoutUnitsComboBox::* )( int )>( &QgsLayoutUnitsComboBox::currentIndexChanged ), this, [ = ]( int )
-  {
-    emit changed( unit() );
-  } );
+  connect( this, static_cast<void ( QgsLayoutUnitsComboBox::* )( int )>( &QgsLayoutUnitsComboBox::currentIndexChanged ), this, &QgsLayoutUnitsComboBox::indexChanged );
 }
 
 QgsUnitTypes::LayoutUnit QgsLayoutUnitsComboBox::unit() const
@@ -50,4 +48,34 @@ void QgsLayoutUnitsComboBox::setUnit( QgsUnitTypes::LayoutUnit unit )
   setCurrentIndex( findData( unit ) );
 }
 
+void QgsLayoutUnitsComboBox::linkToWidget( QDoubleSpinBox *widget )
+{
+  mLinkedSpinBoxes << widget;
+}
+
+void QgsLayoutUnitsComboBox::indexChanged( int )
+{
+  QgsUnitTypes::LayoutUnit newUnit = unit();
+  if ( mConverter )
+  {
+    Q_FOREACH ( const QPointer< QDoubleSpinBox > &widget, mLinkedSpinBoxes )
+    {
+      if ( widget )
+        widget->setValue( mConverter->convert( QgsLayoutMeasurement( widget->value(), mOldUnit ), newUnit ).length() );
+    }
+  }
+  emit changed( newUnit );
+  mOldUnit = newUnit;
+}
+
+QgsLayoutMeasurementConverter *QgsLayoutUnitsComboBox::converter() const
+{
+  return mConverter;
+}
+
+void QgsLayoutUnitsComboBox::setConverter( QgsLayoutMeasurementConverter *converter )
+{
+  mConverter = converter;
+}
+
 #include "qgslayoutunitscombobox.h"

src/gui/layout/qgslayoutunitscombobox.h

@@ -19,6 +19,10 @@
 #include "qgis_gui.h"
 #include "qgis_sip.h"
 #include "qgsunittypes.h"
+#include <QDoubleSpinBox>
+#include <QPointer>
+
+class QgsLayoutMeasurementConverter;
 
 /**
  * \ingroup gui
@@ -50,6 +54,32 @@ class GUI_EXPORT QgsLayoutUnitsComboBox : public QComboBox
      */
     void setUnit( QgsUnitTypes::LayoutUnit unit );
 
+    /**
+     * Registers a spin box \a widget as linked with the combo box.
+     *
+     * Registered spin boxes will automatically be upodated whenever the unit is changed. I.e. a
+     * spin box with a value of 100 will be set to 1 when the unit is changed from centimeters to meters.
+     *
+     * A measurement converter() must be set in order for the automatic unit conversion to occur.
+     *
+     * \see setConverter()
+     */
+    void linkToWidget( QDoubleSpinBox *widget );
+
+    /**
+     * Returns the converter used when automatically converting units for linked widgets.
+     * \see setConverter()
+     */
+    QgsLayoutMeasurementConverter *converter() const;
+
+    /**
+     * Sets a \a converter to use when automatically converting units for linked widgets.
+     * The ownership of \a converter is not transferred, and converter must exist for the
+     * life of the combo box.
+     * \see converter()
+     */
+    void setConverter( QgsLayoutMeasurementConverter *converter );
+
   signals:
 
     /**
@@ -57,6 +87,17 @@ class GUI_EXPORT QgsLayoutUnitsComboBox : public QComboBox
      */
     void changed( QgsUnitTypes::LayoutUnit unit );
 
+  private slots:
+
+    void indexChanged( int index );
+
+  private:
+
+    QgsLayoutMeasurementConverter *mConverter = nullptr;
+
+    QgsUnitTypes::LayoutUnit mOldUnit = QgsUnitTypes::LayoutMillimeters;
+
+    QList< QPointer< QDoubleSpinBox > > mLinkedSpinBoxes;
 };
 
 #endif // QGSLAYOUTUNITSCOMBOBOX_H

src/gui/layout/qgslayoutviewtooladditem.cpp

@@ -90,6 +90,7 @@ void QgsLayoutViewToolAddItem::layoutReleaseEvent( QgsLayoutViewMouseEvent *even
   if ( clickOnly )
   {
     QgsLayoutItemPropertiesDialog dlg( view() );
+    dlg.setLayout( layout() );
     dlg.setItemPosition( QgsLayoutPoint( event->layoutPoint(), layout()->units() ) );
     if ( dlg.exec() )
     {

tests/src/python/test_qgslayoutunitscombobox.py

@@ -14,9 +14,11 @@ __revision__ = '$Format:%H$'
 
 import qgis  # NOQA
 
-from qgis.core import QgsUnitTypes
+from qgis.core import QgsUnitTypes, QgsLayoutMeasurementConverter
 from qgis.gui import QgsLayoutUnitsComboBox
 
+from qgis.PyQt.QtWidgets import QDoubleSpinBox
+
 from qgis.PyQt.QtTest import QSignalSpy
 from qgis.testing import start_app, unittest
 
@@ -42,6 +44,37 @@ class TestQgsLayoutUnitsComboBox(unittest.TestCase):
         self.assertEqual(len(spy), 1)
         self.assertEqual(spy[0][0], QgsUnitTypes.LayoutPixels)
 
+    def testLinkedWidgets(self):
+        """ test linking spin boxes to combobox"""
+        w = qgis.gui.QgsLayoutUnitsComboBox()
+        self.assertFalse(w.converter())
+        c = QgsLayoutMeasurementConverter()
+        w.setConverter(c)
+        self.assertEqual(w.converter(), c)
+
+        spin = QDoubleSpinBox()
+        spin.setMaximum(1000000)
+        spin.setValue(100)
+        w.setUnit(QgsUnitTypes.LayoutCentimeters)
+        w.linkToWidget(spin)
+        w.setUnit(QgsUnitTypes.LayoutMeters)
+        self.assertAlmostEqual(spin.value(), 1.0, 2)
+        w.setUnit(QgsUnitTypes.LayoutMillimeters)
+        self.assertAlmostEqual(spin.value(), 1000.0, 2)
+
+        spin2 = QDoubleSpinBox()
+        spin2.setValue(50)
+        spin2.setMaximum(1000000)
+        w.linkToWidget(spin2)
+        w.setUnit(QgsUnitTypes.LayoutCentimeters)
+        self.assertAlmostEqual(spin.value(), 100.0, 2)
+        self.assertAlmostEqual(spin2.value(), 5.0, 2)
+
+        # no crash!
+        del spin
+        w.setUnit(QgsUnitTypes.LayoutMeters)
+        self.assertAlmostEqual(spin2.value(), 0.05, 2)
+
 
 if __name__ == '__main__':
     unittest.main()