Fix UI mismatch when QPrinter::setDuplex

Amends fa854f214a3c812e5548ff55d179dd07ef99053b. If user call
QPrinter::setDuplex method, the QPrintDialog explicitDuplexMode value
won't update.

Fix this by:
1) handle device-specific default and value set in QPrinter in
QCupsPrintEngine(Private)
2) handle the explicitly user-selected value in QPrintDialog(Private)

Done-with: Michael Weghorn <m.weghorn@posteo.de>
Fixes: QTBUG-99504
Change-Id: I1a471a8554e83aa4bec8bb95fcc95f9135b0ac8c
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
(cherry picked from commit 72931f492092a67c39cd7fb124d54e8752ee4933)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

3 files changed, 13 insertions, 6 deletions

diff --git a/src/plugins/printsupport/cups/qcupsprintengine.cpp b/src/plugins/printsupport/cups/qcupsprintengine.cpp
index f0893df8b5..efe71862ed 100644
--- a/src/plugins/printsupport/cups/qcupsprintengine.cpp
+++ b/src/plugins/printsupport/cups/qcupsprintengine.cpp
@@ -89,8 +89,10 @@ void QCupsPrintEngine::setProperty(PrintEnginePropertyKey key, const QVariant &v
         break;
     case PPK_Duplex: {
         QPrint::DuplexMode mode = QPrint::DuplexMode(value.toInt());
-        if (mode != d->duplex && d->m_printDevice.supportedDuplexModes().contains(mode))
+        if (d->m_printDevice.supportedDuplexModes().contains(mode)) {
             d->duplex = mode;
+            d->duplexRequestedExplicitly = true;
+        }
         break;
     }
     case PPK_PrinterName:
@@ -277,9 +279,12 @@ void QCupsPrintEnginePrivate::changePrinter(const QString &newPrinter)
     m_printDevice.swap(printDevice);
     printerName = m_printDevice.id();
 
-    // Check if new printer supports current settings, otherwise us defaults
-    if (duplex != QPrint::DuplexAuto && !m_printDevice.supportedDuplexModes().contains(duplex))
+    // in case a duplex value was explicitly set, check if new printer supports current value,
+    // otherwise use device default
+    if (!duplexRequestedExplicitly || !m_printDevice.supportedDuplexModes().contains(duplex)) {
         duplex = m_printDevice.defaultDuplexMode();
+        duplexRequestedExplicitly = false;
+    }
     QPrint::ColorMode colorMode = grayscale ? QPrint::GrayScale : QPrint::Color;
     if (!m_printDevice.supportedColorModes().contains(colorMode))
         grayscale = m_printDevice.defaultColorMode() == QPrint::GrayScale;
diff --git a/src/plugins/printsupport/cups/qcupsprintengine_p.h b/src/plugins/printsupport/cups/qcupsprintengine_p.h
index c021b0c643..8f81fe478d 100644
--- a/src/plugins/printsupport/cups/qcupsprintengine_p.h
+++ b/src/plugins/printsupport/cups/qcupsprintengine_p.h
@@ -100,6 +100,7 @@ private:
     QStringList cupsOptions;
     QString cupsTempFile;
     QPrint::DuplexMode duplex;
+    bool duplexRequestedExplicitly = false;
 };
 
 QT_END_NAMESPACE
diff --git a/src/printsupport/dialogs/qprintdialog_unix.cpp b/src/printsupport/dialogs/qprintdialog_unix.cpp
index 7de8ec05f3..dbebd03719 100644
--- a/src/printsupport/dialogs/qprintdialog_unix.cpp
+++ b/src/printsupport/dialogs/qprintdialog_unix.cpp
@@ -708,13 +708,14 @@ void QPrintDialogPrivate::selectPrinter(const QPrinter::OutputFormat outputForma
         else
             options.grayscale->setChecked(true);
 
-        // keep duplex value explicitly set by user, if any, and selected printer supports it;
-        // use device default otherwise
+        // duplex priorities to be as follows:
+        // 1) a user-selected duplex value in the dialog has highest prority
+        // 2) duplex value set in the QPrinter
         QPrint::DuplexMode duplex;
         if (explicitDuplexMode != QPrint::DuplexAuto && supportedDuplexMode.contains(explicitDuplexMode))
             duplex = explicitDuplexMode;
         else
-            duplex = top->d->m_currentPrintDevice.defaultDuplexMode();
+            duplex = static_cast<QPrint::DuplexMode>(p->duplex());
         switch (duplex) {
         case QPrint::DuplexNone:
             options.noDuplex->setChecked(true); break;