Ticket #23584: 23584.5.diff​

Index: wp-content/themes/twentythirteen/style.css
===================================================================
--- wp-content/themes/twentythirteen/style.css  (revision 23615)
+++ wp-content/themes/twentythirteen/style.css  (working copy)
@@ -738,7 +738,8 @@
 }
 
 .wp-caption .wp-caption-text,
-.entry-caption {
+.entry-caption,
+.gallery-caption {
        color: #220e10;
        font-size: 18px;
        font-style: italic;
@@ -1244,6 +1245,8 @@
 .gallery-item {
        float: left;
        margin: 0 4px 4px 0;
+       overflow: hidden;
+       position: relative;
 }
 
 .single .gallery-columns-1.gallery-size-medium,
@@ -1328,14 +1331,22 @@
 }
 
 .gallery-caption {
-       color: #777;
-       font-size: 14px;
-       font-style: italic;
-       padding-top: 4px;
+       background-color: rgba(255, 255, 255, 0.8);
+       margin: 0;
+       overflow: hidden;
+       padding: 2px 10px;
+       position: absolute;
+       bottom: -40px;
+       text-align: left;
+       text-overflow: ellipsis;
+       -webkit-transition: bottom 400ms ease;
+       transition:         bottom 400ms ease;
+       white-space: nowrap;
+       width: 100%;
 }
 
-.gallery-caption {
-       width: 90%;
+.gallery-item:hover .gallery-caption {
+       bottom: 0;
 }