Revise zoom layout shift fix (#66390)

* Contain margins with BFC instead of border

* Move to block canvas styles in the visual editor

---------

Co-authored-by: Andrew Serong <14988353+andrewserong@users.noreply.github.com>
Co-authored-by: stokesman <presstoke@git.wordpress.org>
Co-authored-by: andrewserong <andrewserong@git.wordpress.org>
Co-authored-by: ramonjd <ramonopoly@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

packages/block-editor/src/components/iframe/content.scss

@@ -1,6 +1,5 @@
 .block-editor-iframe__body {
 	position: relative;
-	border: 0.01px solid transparent;
 }
 
 .block-editor-iframe__html {

packages/editor/src/components/visual-editor/index.js

@@ -367,7 +367,10 @@ function VisualEditor( {
 		return [
 			...( styles ?? [] ),
 			{
-				css: `.is-root-container{display:flow-root;${
+				// Ensures margins of children are contained so that the body background paints behind them.
+				// Otherwise, the background of html (when zoomed out) would show there and appear broken. It’s
+				// important mostly for post-only views yet conceivably an issue in templated views too.
+				css: `:where(.block-editor-iframe__body){display:flow-root;}.is-root-container{display:flow-root;${
 					// Some themes will have `min-height: 100vh` for the root container,
 					// which isn't a requirement in auto resize mode.
 					enableResizing ? 'min-height:0!important;' : ''