diff --git a/src/crypto/gui/decryptverifyfilesdialog.cpp b/src/crypto/gui/decryptverifyfilesdialog.cpp
--- a/src/crypto/gui/decryptverifyfilesdialog.cpp
+++ b/src/crypto/gui/decryptverifyfilesdialog.cpp
@@ -42,6 +42,7 @@
 
 #include <Libkleo/FileNameRequester>
 
+#include <QWindow>
 #include <QVBoxLayout>
 #include <QProgressBar>
 #include <QLabel>
@@ -54,6 +55,7 @@
 #include <KMessageBox>
 #include <KConfigGroup>
 #include <KSharedConfig>
+#include <KWindowConfig>
 
 using namespace Kleo;
 using namespace Kleo::Crypto;
@@ -243,18 +245,24 @@
 
 void DecryptVerifyFilesDialog::readConfig()
 {
+    winId(); // ensure there's a window created
+
+    // set default window size
+    windowHandle()->resize(640, 480);
+
+    // restore size from config file
     KConfigGroup cfgGroup(KSharedConfig::openConfig(), "DecryptVerifyFilesDialog");
-    const QByteArray geom = cfgGroup.readEntry("geometry", QByteArray());
-    if (!geom.isEmpty()) {
-        restoreGeometry(geom);
-        return;
-    }
-    resize(QSize(640, 480));
+    KWindowConfig::restoreWindowSize(windowHandle(), cfgGroup);
+
+    // NOTICE: QWindow::setGeometry() does NOT impact the backing QWidget geometry even if the platform
+    // window was created -> QTBUG-40584. We therefore copy the size here.
+    // TODO: remove once this was resolved in QWidget QPA
+    resize(windowHandle()->size());
 }
 
 void DecryptVerifyFilesDialog::writeConfig()
 {
     KConfigGroup cfgGroup(KSharedConfig::openConfig(), "DecryptVerifyFilesDialog");
-    cfgGroup.writeEntry("geometry", saveGeometry());
+    KWindowConfig::saveWindowSize(windowHandle(), cfgGroup);
     cfgGroup.sync();
 }
diff --git a/src/crypto/gui/signencryptfileswizard.h b/src/crypto/gui/signencryptfileswizard.h
--- a/src/crypto/gui/signencryptfileswizard.h
+++ b/src/crypto/gui/signencryptfileswizard.h
@@ -104,6 +104,10 @@
 
     void setLabelText(const QString &label) const;
 
+protected:
+    void readConfig();
+    void writeConfig();
+
 Q_SIGNALS:
     void operationPrepared();
 
diff --git a/src/crypto/gui/signencryptfileswizard.cpp b/src/crypto/gui/signencryptfileswizard.cpp
--- a/src/crypto/gui/signencryptfileswizard.cpp
+++ b/src/crypto/gui/signencryptfileswizard.cpp
@@ -42,13 +42,15 @@
 #include <KSharedConfig>
 #include <KColorScheme>
 #include <KConfigGroup>
+#include <KWindowConfig>
 
 #include <KMessageBox>
 
 #include "kleopatra_debug.h"
 #include <utils/gnupg-helper.h>
 
 #include <Libkleo/FileNameRequester>
+#include <QWindow>
 #include <QVBoxLayout>
 #include <QWizardPage>
 #include <QGroupBox>
@@ -393,6 +395,8 @@
     , mSigningUserMutable(true)
     , mEncryptionUserMutable(true)
 {
+    readConfig();
+
     bool de_vs = Kleo::gpgComplianceP("de-vs");
 #ifdef Q_OS_WIN
     // Enforce modern style to avoid vista style ugliness.
@@ -421,13 +425,6 @@
     } else {
         mLabel = nullptr;
     }
-
-    KConfigGroup cfgGroup(KSharedConfig::openConfig(), "SignEncryptFilesWizard");
-    const QByteArray geom = cfgGroup.readEntry("geometry", QByteArray());
-    if (!geom.isEmpty()) {
-        restoreGeometry(geom);
-        return;
-    }
 }
 
 void SignEncryptFilesWizard::setLabelText(const QString &label) const
@@ -447,9 +444,7 @@
 SignEncryptFilesWizard::~SignEncryptFilesWizard()
 {
     qCDebug(KLEOPATRA_LOG);
-    KConfigGroup cfgGroup(KSharedConfig::openConfig(), "SignEncryptFilesWizard");
-    cfgGroup.writeEntry("geometry", saveGeometry());
-    cfgGroup.sync();
+    writeConfig();
 }
 
 void SignEncryptFilesWizard::setSigningPreset(bool preset)
@@ -518,4 +513,29 @@
     return mSigEncPage->encryptSymmetric();
 }
 
+void SignEncryptFilesWizard::readConfig()
+{
+    winId(); // ensure there's a window created
+
+    // set default window size
+    windowHandle()->resize(640, 480);
+
+    // restore size from config file
+    KConfigGroup cfgGroup(KSharedConfig::openConfig(), "SignEncryptFilesWizard");
+    KWindowConfig::restoreWindowSize(windowHandle(), cfgGroup);
+
+    // NOTICE: QWindow::setGeometry() does NOT impact the backing QWidget geometry even if the platform
+    // window was created -> QTBUG-40584. We therefore copy the size here.
+    // TODO: remove once this was resolved in QWidget QPA
+    resize(windowHandle()->size());
+}
+
+void SignEncryptFilesWizard::writeConfig()
+{
+    KConfigGroup cfgGroup(KSharedConfig::openConfig(), "SignEncryptFilesWizard");
+    KWindowConfig::saveWindowSize(windowHandle(), cfgGroup);
+    cfgGroup.sync();
+}
+
+
 #include "signencryptfileswizard.moc"
diff --git a/src/kleopatraapplication.cpp b/src/kleopatraapplication.cpp
--- a/src/kleopatraapplication.cpp
+++ b/src/kleopatraapplication.cpp
@@ -222,6 +222,10 @@
 
 KleopatraApplication::~KleopatraApplication()
 {
+    // main window doesn't receive "close" signal and cannot
+    // save settings before app exit
+    delete d->mainWindow;
+
     // work around kdelibs bug https://bugs.kde.org/show_bug.cgi?id=162514
     KSharedConfig::openConfig()->sync();
 }
diff --git a/src/mainwindow.cpp b/src/mainwindow.cpp
--- a/src/mainwindow.cpp
+++ b/src/mainwindow.cpp
@@ -59,6 +59,7 @@
 
 #include <KXMLGUIFactory>
 #include <QApplication>
+#include <QSize>
 #include <QLineEdit>
 #include <KActionMenu>
 #include <KActionCollection>
@@ -398,18 +399,19 @@
 
     q->setAcceptDrops(true);
 
+    // set default window size
+    q->resize(QSize(1024, 500));
     q->setAutoSaveSettings();
+
     updateSearchBarClickMessage();
     updateStatusBar();
 }
 
 MainWindow::Private::~Private() {}
 
 MainWindow::MainWindow(QWidget *parent, Qt::WindowFlags flags)
     : KXmlGuiWindow(parent, flags), d(new Private(this))
-{
-    resize(1024, 500);
-}
+{}
 
 MainWindow::~MainWindow() {}
 
@@ -682,26 +684,14 @@
 {
     qCDebug(KLEOPATRA_LOG);
     KXmlGuiWindow::readProperties(cg);
-    savedGeometry = cg.readEntry("savedGeometry", QByteArray());
-    if (!savedGeometry.isEmpty()) {
-        restoreGeometry(savedGeometry);
-    }
-
-    if (! cg.readEntry<bool>("hidden", false)) {
-        show();
-    }
+    setHidden(cg.readEntry("hidden", false));
 }
 
 void MainWindow::saveProperties(KConfigGroup &cg)
 {
     qCDebug(KLEOPATRA_LOG);
     KXmlGuiWindow::saveProperties(cg);
     cg.writeEntry("hidden", isHidden());
-    if (isHidden()) {
-        cg.writeEntry("savedGeometry", savedGeometry);
-    } else {
-        cg.writeEntry("savedGeometry", saveGeometry());
-    }
 }
 
 #include "moc_mainwindow.cpp"