Query Pagination: Correctly position the "next" link on the first pag…

…e. (#42764)

* Move the next link to the right when it's the only one displayed

* add explanatory comment

* use margin-inline-start for RTL

* Add margin-inline-end to the previous link

* restrict CSS to first and last children

packages/block-library/src/query-pagination/style.scss

@@ -13,6 +13,20 @@ $pagination-margin: 0.5em;
 			margin-right: 0;
 		}
 	}
+
+	// This moves the next link to the right side of the container,
+	// which is important when it's the only block displayed
+	// and the block has a "space-between" justification.
+	&.is-content-justification-space-between {
+		> .wp-block-query-pagination-next:last-child {
+			margin-inline-start: auto;
+		}
+		> .wp-block-query-pagination-previous:first-child {
+			margin-inline-end: auto;
+		}
+	}
+
+
 	.wp-block-query-pagination-previous-arrow {
 		margin-right: 1ch;
 		display: inline-block; // Needed so that the transform works.