fix: apply selected paper size after printer settings rebuild

[18202781743]

Summary

• explicitly apply the selected paper size after rebuilding printer settings
• keep the preview and printer page layout consistent when the default item is a custom paper size
• add a regression test for restoring a custom paper-size selection

Tests

• cmake --build build -j6 --target ut-dtkwidget
• print-preview regression test: passed
• cmake --build build6 -j6 --target dtk6widget

Summary by Sourcery

Reapply the selected paper size when printer settings change so custom paper selections remain effective.

Bug Fixes:

• Ensure the selected custom paper size is reapplied after printer settings are rebuilt, keeping the preview and printer page layout consistent.

Tests:

• Add a regression test covering restoration of a custom paper-size selection after changing printers.

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[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: 18202781743

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Details

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• debian/deepin/OWNERS

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[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Ensures the selected custom paper size is explicitly reapplied after printer settings are rebuilt and adds a regression test to lock in this behavior.

Sequence diagram for applying selected paper size after printer change

sequenceDiagram
    actor User
    participant DPrintPreviewDialog
    participant PaperSizeCombo

    User->>DPrintPreviewDialog:_q_printerChanged(index)
    DPrintPreviewDialog->>PaperSizeCombo:blockSignals(true)
    DPrintPreviewDialog->>DPrintPreviewDialog:rebuildPaperSizeList()
    note over DPrintPreviewDialog: current index may not change
    DPrintPreviewDialog->>DPrintPreviewDialog:matchFitablePageSize()
    DPrintPreviewDialog->>DPrintPreviewDialog:marginsUpdate(true)
    DPrintPreviewDialog->>PaperSizeCombo:blockSignals(false)

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File-Level Changes

Change	Details	Files
Ensure selected paper size is reapplied when printer settings are rebuilt during printer change handling.	Call matchFitablePageSize() immediately after rebuilding the paper list in _q_printerChanged to explicitly apply the selected page size when combo-box signals are blocked Keep marginsUpdate(true) and signal unblocking logic intact so layout and behavior remain consistent with existing flow	src/widgets/dprintpreviewdialog.cpp
Add regression coverage to ensure custom paper-size selection is preserved and applied after printer changes.	Extend SPDX copyright range to 2026 to reflect ongoing maintenance Add testPaperSizeAppliedAfterPrinterChanged test to set a custom paper size, trigger printer change (with list rebuild), and assert that both the combo current text and printer pageSize remain on the custom size	tests/testcases/printpreview/ut_dprintpreviewdialog.cpp

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