Fix issue with appender jumping when creating a new pattern. (WordPre…

…ss#59582)

packages/block-editor/src/components/default-block-appender/content.scss

@@ -29,12 +29,15 @@
 	// In "constrained" layout containers, the first and last paragraphs have their margins zeroed out.
 	// In the case of this appender, it needs to apply those same rules to avoid layout shifts.
 	// Such shifts happen when the bottom margin of the Title block has been set to less than the default 1em margin of paragraphs.
-	:where(body .is-layout-constrained) & {
+	:where(body .is-layout-constrained) &,
+	:where(.wp-site-blocks) & {
 		> :first-child:first-child {
 			margin-block-start: 0;
+			margin-block-end: 0;
 		}
 
-		// Since this particular appender will only ever appear on an entirely empty document, we don't account for last-child.
+		// Since this appender will only ever appear on an entirely empty document, we don't account for last-child.
+		// This is also because it will never be the last child, the block inserter that sits in this appender is the last child.
 	}
 
 	// Dropzone.