Fix justification for button block when selected (#33739)

* Remove margin setting, auto right conflict with justify buttons

* Per review, add little margin back

packages/block-editor/src/components/block-list-appender/style.scss

@@ -14,10 +14,9 @@
 		margin: $grid-unit-10 0;
 	}
 
-	// Add an explicit left margin of zero and auto right margin to work in horizontal
-	// flex containers. Without it, a "space-between"-like effect from two auto margins
-	// will cause the black plus to sit in the center of what space is left.
-	margin: 0 auto 0 0;
+	// Add a uniform margin around the block.
+	// This can hopefully avoid havoc in flex containers.
+	margin: $grid-unit-10;
 
 	// Black square plus appender.
 	.block-list-appender__toggle {