Gallery core should be a block element.

src/scss/gallery.scss

@@ -2,6 +2,7 @@
 @use "@material/image-list";
 
 :host {
+  display: block;
   overflow: auto;
   -ms-overflow-style: none; /* Hide scrollbar: IE and Edge */
   scrollbar-width: none;    /* Hide scrollbar: Firefox */