Added missing z-index for hovering images in Files.app's mosaic mode.

chrome/browser/resources/file_manager/css/gallery.css

Index: chrome/browser/resources/file_manager/css/gallery.css
diff --git a/chrome/browser/resources/file_manager/css/gallery.css b/chrome/browser/resources/file_manager/css/gallery.css
index 8f6239f29ce286f1823e03d9b8cf8b9d064809e4..cca99e489dee4bd51b2f715b16be28d5ff002a7b 100644
--- a/chrome/browser/resources/file_manager/css/gallery.css
+++ b/chrome/browser/resources/file_manager/css/gallery.css
@@ -1163,7 +1163,7 @@ body {
     top + bottom + border-top-width + border-bottom-width AND
     left + right + border-left-width + border-right-width */
 .mosaic-tile .img-border {
-  border: 1px solid transparent;  /* Space between the outline and the image. */
+  border: 1px solid black;  /* Space between the outline and the image. */
   bottom: 4px;
   left: 4px;
   outline: 2px solid transparent;
@@ -1180,6 +1180,7 @@ body {
 
 .mosaic[visible='normal'].hover-visible .mosaic-tile:hover {
   -webkit-transform: scale(1.05);
+  z-index: 50;
 }
 
 .mosaic[visible='normal'].hover-visible