fix: apply selected paper size after printer settings rebuild

src/widgets/dprintpreviewdialog.cpp

@@ -1802,6 +1802,9 @@ void DPrintPreviewDialogPrivate::_q_printerChanged(int index)
         }
     }
 
+    // The current index may not change when the paper list is rebuilt, so the
+    // combo-box signal is not guaranteed to apply the selected page size.
+    matchFitablePageSize();
     marginsUpdate(true);
     paperSizeCombo->blockSignals(false);
     if (isInited)

tests/testcases/printpreview/ut_dprintpreviewdialog.cpp

@@ -1,4 +1,4 @@
-// SPDX-FileCopyrightText: 2021 - 2022 UnionTech Software Technology Co., Ltd.
+// SPDX-FileCopyrightText: 2021 - 2026 UnionTech Software Technology Co., Ltd.
 //
 // SPDX-License-Identifier: LGPL-3.0-or-later
 
@@ -338,6 +338,21 @@ TEST_F(ut_DPrintPreviewDialog, testPrintDeviceCombo)
     ASSERT_STREQ(test_dialog_d->pageRangeCombo->currentText().toLocal8Bit(), "All");
 }
 
+TEST_F(ut_DPrintPreviewDialog, testPaperSizeAppliedAfterPrinterChanged)
+{
+    test_dialog_d->paperSizeCombo->setCurrentText("8K");
+    ASSERT_EQ(test_dialog_d->printer->pageSize(), QPrinter::Custom);
+
+    // Rebuild the paper-size list while preserving the selected custom size.
+    // This path blocks the combo-box signal, so the page size must be applied
+    // explicitly after the selection is restored.
+    test_dialog_d->printer->setPageSize(QPageSize::A4);
+    test_dialog_d->_q_printerChanged(test_dialog_d->printDeviceCombo->currentIndex());
+
+    ASSERT_STREQ(test_dialog_d->paperSizeCombo->currentText().toLocal8Bit(), "8K");
+    ASSERT_EQ(test_dialog_d->printer->pageSize(), QPrinter::Custom);
+}
+
 TEST_F(ut_DPrintPreviewDialog, testCancelBtn)
 {
     ASSERT_TRUE(test_dialog_d->cancelBtn->isEnabled());