[improvement] Picker: support Inertial rolling (#3331)

packages/picker/index.less

@@ -55,12 +55,26 @@
     position: absolute;
     top: 50%;
     left: 0;
-    z-index: 1;
+    z-index: 3;
     width: 100%;
     transform: translateY(-50%);
     pointer-events: none;
   }
 
+  &__mask {
+    position: absolute;
+    top: 0;
+    left: 0;
+    z-index: 2;
+    width: 100%;
+    height: 100%;
+    background-image: linear-gradient(180deg, hsla(0, 0%, 100%, .8), hsla(0, 0%, 100%, .6)),
+      linear-gradient(0deg, hsla(0, 0%, 100%, .8), hsla(0, 0%, 100%, .6));
+    background-repeat: no-repeat;
+    background-position: top, bottom;
+    pointer-events: none;
+  }
+
   &-column {
     flex: 1;
     overflow: hidden;
@@ -69,10 +83,9 @@
 
     &__item {
       padding: 0 5px;
-      color: @picker-option-text-color;
+      color: @picker-option-selected-text-color;
 
       &--selected {
-        color: @picker-option-selected-text-color;
         font-weight: 500;
       }