Fix the Increment boxes of the Auto numbering panel cut off on Windows

Each Increment spin box had a fixed maximum width of 55 px. The Windows
11 style puts the up and down arrows side by side, which leaves almost
nothing of 55 px for the number (#1566). The boxes now take the width
their style asks for.

Test: tst_autonumincrementwidth runs the editor under a style that
widens spin boxes the way the Windows 11 style does and checks each
Increment box is at least as wide as the style needs: 55 px against 81
px before this change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/qttest/CMakeLists.txt

@@ -312,6 +312,10 @@ qet_add_app_test(tst_folioimageload tst_folioimageload.cpp)
 # selection has nothing to edit for a moment (#1439).
 qet_add_app_test(tst_selectionpanelheight tst_selectionpanelheight.cpp)
 
+# The Auto numbering panel's Increment spin boxes get the width their style
+# asks for; a fixed 55 px cut the number off on Windows (#1566).
+qet_add_app_test(tst_autonumincrementwidth tst_autonumincrementwidth.cpp)
+
 # Opening a project before the elements panel's first reload() left its tree
 # item detached, and selecting the item's first child crashed in QTreeModel
 # (pre-existing since a82f6de23).

tests/qttest/tst_autonumincrementwidth.cpp

@@ -0,0 +1,64 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include "qetapptest.h"
+
+#include <QDockWidget>
+#include <QProxyStyle>
+#include <QSpinBox>
+
+namespace {
+
+// Spin boxes as wide as the Windows 11 style draws them: its up and down
+// buttons sit side by side, so a spin box needs far more room than with
+// the buttons stacked.
+class WideSpinBoxStyle : public QProxyStyle
+{
+	public:
+		QSize sizeFromContents(ContentsType type, const QStyleOption *option,
+				       const QSize &size, const QWidget *widget) const override
+		{
+			QSize s = QProxyStyle::sizeFromContents(type, option, size, widget);
+			if (type == CT_SpinBox)
+				s.rwidth() += 40;
+			return s;
+		}
+};
+
+}
+
+// The Increment spin boxes of the Auto numbering panel get the width their
+// style asks for (#1566). A fixed 55 px maximum cut off the number on
+// Windows.
+class tst_autonumincrementwidth : public QObject
+{
+	Q_OBJECT
+
+	private slots:
+		void incrementFitsItsStyle()
+		{
+			QApplication::setStyle(new WideSpinBoxStyle);
+
+			QETDiagramEditor *editor = QetAppTest::editor();
+			QVERIFY(editor);
+			editor->resize(1600, 1000);
+			editor->show();
+			QVERIFY(QTest::qWaitForWindowExposed(editor));
+
+			const QStringList names{"m_conductor_increase_sb",
+						"m_element_increase_sb",
+						"m_folio_increase_sb"};
+			for (const QString &name : names) {
+				auto spin_box = editor->findChild<QSpinBox *>(name);
+				QVERIFY2(spin_box, qPrintable(name));
+				QVERIFY2(spin_box->isVisible(), qPrintable(name));
+				QVERIFY2(spin_box->width() >= spin_box->minimumSizeHint().width(),
+					 qPrintable(QString("%1 is %2 px, its style needs %3 px")
+						    .arg(name)
+						    .arg(spin_box->width())
+						    .arg(spin_box->minimumSizeHint().width())));
+			}
+		}
+};
+
+QET_APP_TEST_MAIN(tst_autonumincrementwidth)
+
+#include "tst_autonumincrementwidth.moc"