diff --git a/src/Gui/PreferencePages/DlgSettingsGeneral.cpp b/src/Gui/PreferencePages/DlgSettingsGeneral.cpp
index 0e93f92b1..732457a64 100644
--- a/src/Gui/PreferencePages/DlgSettingsGeneral.cpp
+++ b/src/Gui/PreferencePages/DlgSettingsGeneral.cpp
@@ -150,7 +150,9 @@ void DlgSettingsGeneral::saveUnitSystemSettings()
     ParameterGrp::handle hGrpu = App::GetApplication().GetParameterGroupByPath(
         "User parameter:BaseApp/Preferences/Units"
     );
-    hGrpu->SetInt("UserSchema", ui->comboBox_UnitSystem->currentIndex());
+    const int selectedSchema = ui->comboBox_UnitSystem->currentIndex();
+    const bool schemaChanged = selectedSchema != hGrpu->GetInt("UserSchema", 0);
+    hGrpu->SetInt("UserSchema", selectedSchema);
     hGrpu->SetInt("Decimals", ui->spinBoxDecimals->value());
     hGrpu->SetBool("IgnoreProjectSchema", ui->checkBox_projectUnitSystemIgnore->isChecked());
 
@@ -171,20 +173,26 @@ void DlgSettingsGeneral::saveUnitSystemSettings()
     // Set the actual format value
     UnitsApi::setDenominator(FracInch);
 
-    // Set and save the Unit System
-    if (ui->checkBox_projectUnitSystemIgnore->isChecked()) {
-        // currently selected View System (unit system)
-        int viewSystemIndex = ui->comboBox_UnitSystem->currentIndex();
-        UnitsApi::setSchema(viewSystemIndex);
-    }
-    else if (App::Document* doc = App::GetApplication().getActiveDocument()) {
-        UnitsApi::setSchema(doc->UnitSystem.getValue());
-    }
-    else {
-        // if there is no existing document then the unit must still be set
-        int viewSystemIndex = ui->comboBox_UnitSystem->currentIndex();
-        UnitsApi::setSchema(viewSystemIndex);
-    }
+    // Set and save the Unit System
+    if (ui->checkBox_projectUnitSystemIgnore->isChecked()) {
+        // Use the preference for the current view without changing the
+        // document's own unit system.
+        UnitsApi::setSchema(selectedSchema);
+    }
+    else if (App::Document* doc = App::GetApplication().getActiveDocument()) {
+        if (schemaChanged) {
+            getMainWindow()->setUserSchema(selectedSchema);
+        }
+        else {
+            // Applying unrelated preferences or restoring project units must
+            // preserve the document's own schema.
+            UnitsApi::setSchema(doc->UnitSystem.getValue());
+        }
+    }
+    else {
+        // if there is no existing document then the unit must still be set
+        UnitsApi::setSchema(selectedSchema);
+    }
 
     ui->SubstituteDecimal->onSave();
     ui->UseLocaleFormatting->onSave();
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index edf27339e..56c7f67d0 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -109,6 +109,12 @@ if(WIN32)
     elseif(EXISTS "${CMAKE_PREFIX_PATH}/bin")
         list(APPEND _test_env_mod "PATH=path_list_prepend:${CMAKE_PREFIX_PATH}/bin")
     endif()
+    set(_qt_platform_plugin_path "${CMAKE_PREFIX_PATH}/lib/qt6/plugins/platforms")
+    if(EXISTS "${_qt_platform_plugin_path}")
+        list(APPEND _test_env_mod
+            "QT_QPA_PLATFORM_PLUGIN_PATH=set:${_qt_platform_plugin_path}"
+        )
+    endif()
     file(GLOB _mod_dirs LIST_DIRECTORIES true "${CMAKE_BINARY_DIR}/Mod/*")
     foreach(_mod_dir ${_mod_dirs})
         if(IS_DIRECTORY "${_mod_dir}")
diff --git a/tests/src/Gui/CMakeLists.txt b/tests/src/Gui/CMakeLists.txt
index 78a7dc055..b914eb61d 100644
--- a/tests/src/Gui/CMakeLists.txt
+++ b/tests/src/Gui/CMakeLists.txt
@@ -29,6 +29,11 @@ setup_qt_test(MacroTaskAcceptance)
 setup_qt_test(QuantitySpinBox)
 set(RenderMeshController_LIBS PartGui Part)
 setup_qt_test(RenderMeshController)
+setup_qt_test(UnitSettings)
+
+target_sources(UnitSettings_Tests_run PRIVATE
+    ${CMAKE_SOURCE_DIR}/src/Gui/PreferencePages/DlgSettingsGeneral.cpp
+)
 
 target_link_libraries(Gui_tests_run PRIVATE
     GTest::gmock_main
diff --git a/tests/src/Gui/UnitSettings.cpp b/tests/src/Gui/UnitSettings.cpp
new file mode 100644
index 000000000..de898e5d8
--- /dev/null
+++ b/tests/src/Gui/UnitSettings.cpp
@@ -0,0 +1,142 @@
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#include <memory>
+#include <string>
+
+#include <QCheckBox>
+#include <QComboBox>
+#include <QTest>
+
+#include <App/Application.h>
+#include <App/Document.h>
+#include <Base/Parameter.h>
+#include <Base/UnitsApi.h>
+#include <Gui/Application.h>
+#include <Gui/MainWindow.h>
+#include <src/App/InitApplication.h>
+
+#include <Gui/PreferencePages/DlgSettingsGeneral.h>
+
+class UnitSettingsTest: public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void initTestCase()
+    {
+        tests::initApplication();
+        Gui::Application::initApplication();
+        Gui::Application::initOpenInventor();
+        guiApplication = std::make_unique<Gui::Application>(true);
+        mainWindow = std::make_unique<Gui::MainWindow>();
+    }
+
+    void cleanup()
+    {
+        if (!documentName.empty()
+            && App::GetApplication().getDocument(documentName.c_str())) {
+            App::GetApplication().closeDocument(documentName.c_str());
+            documentName.clear();
+        }
+        preferences()->SetInt("UserSchema", originalSchema);
+        preferences()->SetBool("IgnoreProjectSchema", originalIgnoreProjectSchema);
+        Base::UnitsApi::setSchema(originalSchema);
+    }
+
+    void cleanupTestCase()
+    {
+        // FreeCAD's GUI singleton is process-scoped; leave it alive until the
+        // test process exits rather than tearing it down after QTest.
+        mainWindow.release();
+        guiApplication.release();
+    }
+
+    void savingUnitPreferences_data()
+    {
+        QTest::addColumn<int>("projectSchema");
+        QTest::addColumn<int>("selectedSchema");
+        QTest::addColumn<bool>("initialIgnore");
+        QTest::addColumn<bool>("ignoreProject");
+        QTest::addColumn<int>("expectedProjectSchema");
+        QTest::addColumn<int>("expectedViewSchema");
+
+        QTest::newRow("change-open-document") << 0 << 3 << false << false << 3 << 3;
+        QTest::newRow("unchanged-preference") << 3 << 0 << false << false << 3 << 3;
+        QTest::newRow("ignore-project") << 0 << 3 << false << true << 0 << 3;
+        QTest::newRow("ignore-project-unchanged") << 3 << 0 << true << true << 3 << 0;
+        QTest::newRow("restore-project-units") << 3 << 0 << true << false << 3 << 3;
+        QTest::newRow("change-and-restore-project-units") << 0 << 3 << true << false << 3 << 3;
+        QTest::newRow("no-document") << -1 << 3 << false << false << -1 << 3;
+    }
+
+    void savingUnitPreferences()
+    {
+        QFETCH(int, projectSchema);
+        QFETCH(int, selectedSchema);
+        QFETCH(bool, initialIgnore);
+        QFETCH(bool, ignoreProject);
+        QFETCH(int, expectedProjectSchema);
+        QFETCH(int, expectedViewSchema);
+        auto units = preferences();
+        originalSchema = units->GetInt("UserSchema", 0);
+        originalIgnoreProjectSchema = units->GetBool("IgnoreProjectSchema", false);
+        units->SetInt("UserSchema", 0);
+        units->SetBool("IgnoreProjectSchema", initialIgnore);
+        Base::UnitsApi::setSchema(0);
+
+        App::Document* document = nullptr;
+        if (projectSchema >= 0) {
+            documentName = App::GetApplication().getUniqueDocumentName("unit_settings_test");
+            document = App::GetApplication().newDocument(documentName.c_str());
+            QVERIFY(document != nullptr);
+            document->UnitSystem.setValue(static_cast<long>(projectSchema));
+
+            auto* guiDocument = guiApplication->getDocument(document);
+            QVERIFY(guiDocument != nullptr);
+            guiApplication->setActiveDocument(guiDocument);
+        }
+
+        Gui::Dialog::DlgSettingsGeneral dialog;
+        dialog.loadSettings();
+        auto* unitSystem = dialog.findChild<QComboBox*>(QStringLiteral("comboBox_UnitSystem"));
+        QVERIFY(unitSystem != nullptr);
+        QVERIFY(unitSystem->count() > selectedSchema);
+        unitSystem->setCurrentIndex(selectedSchema);
+        auto* ignore = dialog.findChild<QCheckBox*>(
+            QStringLiteral("checkBox_projectUnitSystemIgnore")
+        );
+        QVERIFY(ignore != nullptr);
+        ignore->setChecked(ignoreProject);
+
+        dialog.saveSettings();
+
+        if (document) {
+            QCOMPARE(document->UnitSystem.getValue(), expectedProjectSchema);
+        }
+        QCOMPARE(units->GetInt("UserSchema", 0), selectedSchema);
+        QCOMPARE(units->GetBool("IgnoreProjectSchema", false), ignoreProject);
+        const auto translated = Base::UnitsApi::schemaTranslate(Base::Quantity(25.4, "mm"));
+        QCOMPARE(
+            QString::fromStdString(translated).contains(QStringLiteral("in")),
+            expectedViewSchema == 3
+        );
+    }
+
+private:
+    ParameterGrp::handle preferences() const
+    {
+        return App::GetApplication().GetParameterGroupByPath(
+            "User parameter:BaseApp/Preferences/Units"
+        );
+    }
+
+    std::unique_ptr<Gui::Application> guiApplication;
+    std::unique_ptr<Gui::MainWindow> mainWindow;
+    std::string documentName;
+    int originalSchema {0};
+    bool originalIgnoreProjectSchema {false};
+};
+
+QTEST_MAIN(UnitSettingsTest)
+
+#include "UnitSettings.moc"