enable slow shadow.

Runtime/gestures/converter.cs

                 switch (datum.change) {
                     case PointerChange.down: {
                         _PointerState state = _ensureStateForPointer(datum, position);
-                        D.assert(!state.down);
+                        if (state.down) {
+                            break;
+                        }
                         if (state.lastPosition != position) {
                             // a hover event to be here.
                             state.lastPosition = position;

Runtime/ui/painting/shadow_utils.cs

 
 namespace Unity.UIWidgets.ui {
     static class ShadowUtils {
-        public static bool kUseFastShadow = true;
+        public static bool kUseFastShadow = false;
 
         public const float kAmbientHeightFactor = 1.0f / 128.0f;
         public const float kAmbientGeomFactor = 64.0f;