XWIKI-21756: Missing border at the bottom of docextrapanes (#2787)

xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/less/layout.less

@@ -321,12 +321,12 @@ already take care of padding and shadow. */
 }
 
 #docextrapanes {
-  border-top: 1px solid @xwiki-border-color;
+  border: 1px solid @xwiki-border-color;
   border-top-left-radius: @border-radius-base;
   border-top-right-radius: @border-radius-base;
   margin-top: -1px;
-  border-left: 1px solid @xwiki-border-color;
-  border-right: 1px solid @xwiki-border-color;
+  /* Makes the bottom border hidden below the footer on pages with a long content (longer than the viewport height). */
+  margin-bottom: -1px;
 }
 
 #xdocFooter {