kcms/libkwindevices: Fix serializeMatrix serializing in the wrong order

We were switching which order the columns/rows were in accidentally,
this should be fixed in lock-step with KWin because the same code exists
there.

I copied over the simple test from KWin too to verify this actually
works as expected now.

kcms/libkwindevices/CMakeLists.txt

@@ -23,3 +23,7 @@ target_link_libraries(kwindevices
     Qt::DBus
     Qt::Gui
 )
+
+if(BUILD_TESTING)
+    add_subdirectory(tests)
+endif()

kcms/libkwindevices/inputdevice.cpp

@@ -139,10 +139,12 @@ bool InputDevice::pressureCurveIsDefault() const
 QString InputDevice::serializeMatrix(const QMatrix4x4 &matrix)
 {
     QString result;
-    for (int i = 0; i < 16; i++) {
-        result.append(QString::number(matrix.constData()[i]));
-        if (i != 15) {
-            result.append(QLatin1Char(','));
+    for (int i = 0; i < 4; ++i) {
+        for (int j = 0; j < 4; ++j) {
+            if (!result.isEmpty()) {
+                result.append(QLatin1Char(','));
+            }
+            result.append(QString::number(matrix(i, j)));
         }
     }
     return result;

kcms/libkwindevices/inputdevice.h

@@ -17,6 +17,8 @@
 
 #include "kwindevices-logging.h"
 
+class InputDeviceTest;
+
 namespace KWinDevices
 {
 
@@ -261,6 +263,12 @@ Q_SIGNALS:
     void relativeChanged();
     void currentModesChanged();
 
+protected:
+    static QString serializeMatrix(const QMatrix4x4 &matrix);
+    static QMatrix4x4 deserializeMatrix(const QString &matrix);
+
+    friend class ::InputDeviceTest;
+
 private:
     template<typename T>
     struct Prop {
@@ -384,9 +392,6 @@ private:
         mutable std::optional<T> m_value;
     };
 
-    static QString serializeMatrix(const QMatrix4x4 &matrix);
-    static QMatrix4x4 deserializeMatrix(const QString &matrix);
-
     //
     // general
     Prop<QString> m_name = Prop<QString>(this, "name");

kcms/libkwindevices/tests/CMakeLists.txt

@@ -0,0 +1,4 @@
+ecm_add_test(
+        TEST_NAME tst_inputdevice
+        tst_inputdevice.cpp tst_inputdevice.cpp)
+target_link_libraries(tst_inputdevice PRIVATE Qt::Test kwindevices)

kcms/libkwindevices/tests/tst_inputdevice.cpp

@@ -0,0 +1,31 @@
+/*
+    SPDX-FileCopyrightText: 2026 Joshua Goins <josh@redstrate.com>
+
+    SPDX-License-Identifier: GPL-2.0-or-later
+*/
+
+#include <QTest>
+
+#include "../inputdevice.h"
+
+#include <QSignalSpy>
+
+using namespace KWinDevices;
+
+class InputDeviceTest : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    // Ensure we can actually do a round-trip serialization of matrices.
+    void testMatrix()
+    {
+        QMatrix4x4 matrix{-1.0f, 0.0f, 1.0f, 0.0f, 0.0f, -1.0f, 1.0f, 0.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, 0.0f, 1.0f};
+
+        QCOMPARE(InputDevice::deserializeMatrix(InputDevice::serializeMatrix(matrix)), matrix);
+    }
+};
+
+QTEST_MAIN(InputDeviceTest)
+
+#include "tst_inputdevice.moc"

kcms/tablet/tests/CMakeLists.txt

@@ -2,4 +2,4 @@ ecm_add_test(
         TEST_NAME tst_calibrationtool
         tst_calibrationtool.cpp ../calibrationtool.cpp ${CMAKE_CURRENT_BINARY_DIR}/../logging.cpp)
 target_link_libraries(tst_calibrationtool PRIVATE Qt::Test KF6::ConfigCore Canberra::Canberra kwindevices)
-target_include_directories(tst_calibrationtool PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/..) # ECM logging from the KCM
+target_include_directories(tst_calibrationtool PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/..) # ECM logging from the KCM