Fix scene move gizmo axis projection

tests/editor/test_scene_viewport_move_gizmo.cpp

@@ -7,6 +7,10 @@
 namespace XCEngine::Editor {
 namespace {
 
+float HandleLength(const SceneViewportMoveGizmoHandleDrawData& handle) {
+    return (handle.end - handle.start).Magnitude();
+}
+
 class SceneViewportMoveGizmoTest : public ::testing::Test {
 protected:
     void SetUp() override {
@@ -26,6 +30,19 @@ protected:
         return overlay;
     }
 
+    static SceneViewportOverlayData MakeDownwardTiltedOverlay() {
+        SceneViewportOverlayData overlay = {};
+        overlay.valid = true;
+        overlay.cameraPosition = Math::Vector3(0.0f, 10.0f, -1.0f);
+        overlay.cameraForward = (Math::Vector3::Zero() - overlay.cameraPosition).Normalized();
+        overlay.cameraRight = Math::Vector3::Cross(Math::Vector3::Up(), overlay.cameraForward).Normalized();
+        overlay.cameraUp = Math::Vector3::Cross(overlay.cameraForward, overlay.cameraRight).Normalized();
+        overlay.verticalFovDegrees = 60.0f;
+        overlay.nearClipPlane = 0.03f;
+        overlay.farClipPlane = 2000.0f;
+        return overlay;
+    }
+
     static SceneViewportMoveGizmoContext MakeContext(
         Components::GameObject* selectedObject,
         const Math::Vector2& mousePosition) {
@@ -37,6 +54,18 @@ protected:
         return context;
     }
 
+    static SceneViewportMoveGizmoContext MakeContext(
+        Components::GameObject* selectedObject,
+        const Math::Vector2& mousePosition,
+        const SceneViewportOverlayData& overlay) {
+        SceneViewportMoveGizmoContext context = {};
+        context.overlay = overlay;
+        context.viewportSize = Math::Vector2(800.0f, 600.0f);
+        context.mousePosition = mousePosition;
+        context.selectedObject = selectedObject;
+        return context;
+    }
+
     SceneManager& GetSceneManager() {
         return dynamic_cast<SceneManager&>(m_context.GetSceneManager());
     }
@@ -92,5 +121,23 @@ TEST_F(SceneViewportMoveGizmoTest, DraggingXAxisOnlyChangesWorldXAndCreatesUndoS
     EXPECT_NEAR(restoredPosition.z, 0.0f, 1e-4f);
 }
 
+TEST_F(SceneViewportMoveGizmoTest, AxisNearlyFacingCameraShrinksInsteadOfKeepingFullLength) {
+    Components::GameObject* target = GetSceneManager().CreateEntity("Target");
+    ASSERT_NE(target, nullptr);
+
+    SceneViewportMoveGizmo gizmo;
+    gizmo.Update(MakeContext(target, Math::Vector2(400.0f, 300.0f), MakeDownwardTiltedOverlay()));
+
+    ASSERT_TRUE(gizmo.GetDrawData().visible);
+    const float xLength = HandleLength(gizmo.GetDrawData().handles[0]);
+    const float yLength = HandleLength(gizmo.GetDrawData().handles[1]);
+    const float zLength = HandleLength(gizmo.GetDrawData().handles[2]);
+
+    EXPECT_GT(xLength, 60.0f);
+    EXPECT_GT(zLength, 60.0f);
+    EXPECT_LT(yLength, xLength * 0.5f);
+    EXPECT_LT(yLength, zLength * 0.5f);
+}
+
 } // namespace
 } // namespace XCEngine::Editor