diff --git a/src/widgets/dialogs/qmessagebox.cpp b/src/widgets/dialogs/qmessagebox.cpp
index dcddc693c83..3350a926f0c 100644
--- a/src/widgets/dialogs/qmessagebox.cpp
+++ b/src/widgets/dialogs/qmessagebox.cpp
@@ -2565,8 +2565,10 @@ void QMessageBox::setDetailedText(const QString &text)
             d->detailsText->hide();
         }
         if (!d->detailsButton) {
+            const bool autoAddOkButton = d->autoAddOkButton; // QTBUG-39334, addButton() clears the flag.
             d->detailsButton = new DetailButton(this);
             addButton(d->detailsButton, QMessageBox::ActionRole);
+            d->autoAddOkButton = autoAddOkButton;
         }
         d->detailsText->setText(text);
     }
diff --git a/tests/auto/widgets/dialogs/qmessagebox/tst_qmessagebox.cpp b/tests/auto/widgets/dialogs/qmessagebox/tst_qmessagebox.cpp
index 0425db30986..f9abd50a154 100644
--- a/tests/auto/widgets/dialogs/qmessagebox/tst_qmessagebox.cpp
+++ b/tests/auto/widgets/dialogs/qmessagebox/tst_qmessagebox.cpp
@@ -593,6 +593,11 @@ void tst_QMessageBox::detailsText()
     QString text("This is the details text.");
     box.setDetailedText(text);
     QCOMPARE(box.detailedText(), text);
+    box.show();
+    QTest::qWaitForWindowExposed(&box);
+    // QTBUG-39334, the box should now have the default "Ok" button as well as
+    // the "Show Details.." button.
+    QCOMPARE(box.findChildren<QAbstractButton *>().size(), 2);
 }
 
 void tst_QMessageBox::detailsButtonText()