add min-height for fullscreen modal

less/modal.less

@@ -31,6 +31,10 @@
   border-radius: 0 0 @modal-dialog-border-radius @modal-dialog-border-radius;
 }
 
+.hook-modal-full() {
+  min-height: 100vh;
+}
+
 .hook-modal-misc() {
   .uk-modal, uk-dropdown {