Files.app: Calculate the hit elements on drag-selection correctly.

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

 /* Copyright (c) 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 /* Special attribute used in HTML to hide elements. */
 body[type='folder'] [invisibleif~='folder'],
 body[type='upload-folder'] [invisibleif~='upload-folder'],
 body[type='saveas-file'] [invisibleif~='saveas-file'],
 body[type='open-file'] [invisibleif~='open-file'],
 body[type='open-multi-file'] [invisibleif~='open-multi-file'],
@@ skipping 951 matching lines @@
 .volume-warning .drive-text {
   flex: none;
   margin-right: 11px;
 }
 
 /* The cr.ui.Grid representing the detailed file list. */
 .thumbnail-grid {
   /* On the right side, we have less margin to pack items as long as they are
      fully visible. */
   -webkit-padding-end: 2px;
-  -webkit-padding-start: 7px;
+  -webkit-padding-start: 4px;
   box-sizing: border-box;
   overflow-y: auto;
-  padding-bottom: 7px;
-  padding-top: 7px;
+  padding-bottom: 16px;
+  padding-top: 4px;
   width: 100%;
 }
 
 body[type='full-page'] .thumbnail-frame > .img-container {
   position: relative;
 }
 
 body[type='full-page'] .thumbnail-frame > .img-container,
 body[type='full-page'] .detail-name .detail-icon {
   cursor: pointer;
@@ skipping 23 matching lines @@
 }
 
 /* Styles specific for the grid view. */
 
 .thumbnail-grid .thumbnail-item {
   -webkit-margin-start: 12px;
   background-color: rgb(245, 245, 245);
   border-radius: 2px;
   box-shadow: 0 1px 4px 0 rgba(0, 0, 0, 0.37);
   height: 180px;
-  margin-top: 8px;
+  margin-top: 12px;
   overflow: hidden;
   position: relative;
   transition: box-shadow 220ms ease;
+  vertical-align: top;  /* Prevent vertical spacing for wrapped inline box. */
   width: 180px;
 }
 
 .thumbnail-grid .thumbnail-item .thumbnail {
   border-radius: 2px;
 }
 
 .thumbnail-grid .thumbnail-frame {
   height: 100%;
   width: 100%;
@@ skipping 987 matching lines @@
   padding: 20px 15px;
 }
 
 .text-measure {
   pointer-events: none;
   position: absolute;
   top: 0;
   visibility: hidden;
   z-index: -1;
 }