Files.app: Add fade animation on toggling checkmarks on grid view.

ui/file_manager/file_manager/foreground/css/file_manager.css

Index: ui/file_manager/file_manager/foreground/css/file_manager.css
diff --git a/ui/file_manager/file_manager/foreground/css/file_manager.css b/ui/file_manager/file_manager/foreground/css/file_manager.css
index e79b26e433c5888e141dcdf586066a64aaa2c73b..2558289ae451b7b1cc98e971a557a56e27a6250d 100644
--- a/ui/file_manager/file_manager/foreground/css/file_manager.css
+++ b/ui/file_manager/file_manager/foreground/css/file_manager.css
@@ -1039,24 +1039,32 @@ body[type='full-page'] .detail-name .detail-icon {
   width: 34px;
 }
 
-body.selecting .thumbnail-grid .checkmark {
+.thumbnail-grid .checkmark.inactive {
   background-image: -webkit-image-set(
       url(../images/files/ui/thumbnail_unchecked.png) 1x,
       url(../images/files/ui/2x/thumbnail_unchecked.png) 2x);
-  opacity: 0.6;
 }
 
-body.check-select .thumbnail-grid .thumbnail-item .checkmark {
+.thumbnail-grid .checkmark.active {
   background-image: -webkit-image-set(
-      url(../images/files/ui/thumbnail_unchecked.png) 1x,
-      url(../images/files/ui/2x/thumbnail_unchecked.png) 2x);
+      url(../images/files/ui/thumbnail_checked.png) 1x,
+      url(../images/files/ui/2x/thumbnail_checked.png) 2x);
+}
+
+body.selecting .thumbnail-grid .checkmark.inactive {
+  opacity: 0.6;
+}
+
+body.check-select .thumbnail-grid .thumbnail-item .checkmark.inactive {
   opacity: 1;
 }
 
-body.check-select .thumbnail-grid .thumbnail-item[selected] .checkmark {
-  background-image: -webkit-image-set(
-      url(../images/files/ui/thumbnail_checked.png) 1x,
-      url(../images/files/ui/2x/thumbnail_checked.png) 2x);
+body.check-select .thumbnail-grid .thumbnail-item[selected]
+    .checkmark.inactive {
+  opacity: 0;
+}
+
+body.check-select .thumbnail-grid .thumbnail-item[selected] .checkmark.active {
   opacity: 1;
 }