Fix Paragraph appender layout shift (building on 66061) (#66779)

* Style to prevent Layout shift when placeholder is clicked

* Refactor style placement to target block-appender__content specifically

* Remove no longer needed rules

---------

Co-authored-by: Vrishabhsk <vrishabhsk@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>
Co-authored-by: jasmussen <joen@git.wordpress.org>

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

@@ -24,20 +24,11 @@
 	.block-editor-default-block-appender__content {
 		// Set the opacity of the initial block appender to the same as placeholder text in an empty Paragraph block.
 		opacity: 0.62;
-	}
-
-	// 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(.wp-site-blocks) & {
-		> :first-child {
-			margin-block-start: 0;
-			margin-block-end: 0;
-		}
 
-		// 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.
+		// The following prevents user agent styles from applying margins to the appender's inner paragraph.
+		// This in turn prevents layout shift due to layout styles removing margins from first and last children.
+		margin-block-start: 0;
+		margin-block-end: 0;
 	}
 
 	// Dropzone.