Position scaled html within available container space

packages/base-styles/_animations.scss

@@ -36,11 +36,6 @@
 	@include reduce-motion("animation");
 }
 
-@mixin editor-canvas-resize-animation() {
-	transition: all 0.4s cubic-bezier(0.46, 0.03, 0.52, 0.96);
-	@include reduce-motion("transition");
-}
-
 // Deprecated
 @mixin edit-post__fade-in-animation($speed: 0.08s, $delay: 0s) {
 	@warn "The `edit-post__fade-in-animation` mixin is deprecated. Use `animation__fade-in` instead.";

packages/block-editor/src/components/block-canvas/style.scss

@@ -4,5 +4,4 @@ iframe[name="editor-canvas"] {
 	height: 100%;
 	display: block;
 	background-color: transparent;
-	@include editor-canvas-resize-animation;
 }

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

@@ -5,18 +5,33 @@
 
 .block-editor-iframe__html {
 	transform-origin: top center;
-	@include editor-canvas-resize-animation;
+	// 400ms should match the animation speed used in iframe/index.js
+	$zoomOutAnimation: all 400ms cubic-bezier(0.46, 0.03, 0.52, 0.96);
+
+	// We don't want to animate the transform of the translateX because it is used
+	// to "center" the canvas. Leaving it on causes the canvas to slide around in
+	// odd ways.
+	transition: $zoomOutAnimation, transform 0s scale 0s;
+	@include reduce-motion("transition");
+
+	&.zoom-out-animation {
+		// we only want to animate the scaling when entering zoom out. When sidebars
+		// are toggled, the resizing of the iframe handles scaling the canvas as well,
+		// and the doubled animations cause very odd animations.
+		transition: $zoomOutAnimation, transform 0s;
+	}
 }
 
 .block-editor-iframe__html.is-zoomed-out {
 	$scale: var(--wp-block-editor-iframe-zoom-out-scale);
 	$frame-size: var(--wp-block-editor-iframe-zoom-out-frame-size);
 	$inner-height: var(--wp-block-editor-iframe-zoom-out-inner-height);
 	$content-height: var(--wp-block-editor-iframe-zoom-out-content-height);
-	$prev-container-width: var(--wp-block-editor-iframe-zoom-out-prev-container-width);
-
-	transform: scale(#{$scale});
-
+	$outer-container-width: var(--wp-block-editor-iframe-zoom-out-outer-container-width);
+	$container-width: var(--wp-block-editor-iframe-zoom-out-container-width, 100vw);
+	// Apply an X translation to center the scaled content within the available space.
+	transform: translateX(calc(( #{$outer-container-width} - #{ $container-width }) / 2 / #{$scale}));
+	scale: #{$scale};
 	background-color: $gray-300;
 
 	// Chrome seems to respect that transform scale shouldn't affect the layout size of the element,

packages/block-editor/src/components/iframe/index.js

@@ -122,7 +122,7 @@ function Iframe( {
 	}, [] );
 	const { styles = '', scripts = '' } = resolvedAssets;
 	const [ iframeDocument, setIframeDocument ] = useState();
-	const prevContainerWidthRef = useRef();
+	const initialContainerWidth = useRef();
 	const [ bodyClasses, setBodyClasses ] = useState( [] );
 	const clearerRef = useBlockSelectionClearer();
 	const [ before, writingFlowRef, after ] = useWritingFlow();
@@ -243,7 +243,7 @@ function Iframe( {
 
 	useEffect( () => {
 		if ( ! isZoomedOut ) {
-			prevContainerWidthRef.current = containerWidth;
+			initialContainerWidth.current = containerWidth;
 		}
 	}, [ containerWidth, isZoomedOut ] );
 
@@ -298,14 +298,49 @@ function Iframe( {
 
 	useEffect( () => cleanup, [ cleanup ] );
 
+	const zoomOutAnimationClassnameRef = useRef( null );
+	const handleZoomOutAnimationClassname = () => {
+		clearTimeout( zoomOutAnimationClassnameRef.current );
+
+		iframeDocument.documentElement.classList.add( 'zoom-out-animation' );
+
+		zoomOutAnimationClassnameRef.current = setTimeout( () => {
+			iframeDocument.documentElement.classList.remove(
+				'zoom-out-animation'
+			);
+		}, 400 ); // 400ms should match the animation speed used in components/iframe/content.scss
+	};
+
+	// Toggle zoom out CSS Classes only when zoom out mode changes. We could add these into the useEffect
+	// that controls settings the CSS variables, but then we would need to do more work to ensure we're
+	// only toggling these when the zoom out mode changes, as that useEffect is also triggered by a large
+	// number of dependencies.
 	useEffect( () => {
 		if ( ! iframeDocument || ! isZoomedOut ) {
 			return;
 		}
 
+		handleZoomOutAnimationClassname();
 		iframeDocument.documentElement.classList.add( 'is-zoomed-out' );
 
+		return () => {
+			handleZoomOutAnimationClassname();
+			iframeDocument.documentElement.classList.remove( 'is-zoomed-out' );
+		};
+	}, [ iframeDocument, isZoomedOut ] );
+
+	// Calculate the scaling and CSS variables for the zoom out canvas
+	useEffect( () => {
+		if ( ! iframeDocument || ! isZoomedOut ) {
+			return;
+		}
+
 		const maxWidth = 750;
+		// Note: When we initialize the zoom out when the canvas is smaller (sidebars open),
+		// initialContainerWidth will be smaller than the full page, and reflow will happen
+		// when the canvas area becomes larger due to sidebars closing. This is a known but
+		// minor divergence for now.
+
 		// This scaling calculation has to happen within the JS because CSS calc() can
 		// only divide and multiply by a unitless value. I.e. calc( 100px / 2 ) is valid
 		// but calc( 100px / 2px ) is not.
@@ -314,7 +349,10 @@ function Iframe( {
 			scale === 'default'
 				? ( Math.min( containerWidth, maxWidth ) -
 						parseInt( frameSize ) * 2 ) /
-						prevContainerWidthRef.current
+						Math.max(
+							initialContainerWidth.current,
+							containerWidth
+						)
 				: scale
 		);
 
@@ -336,13 +374,16 @@ function Iframe( {
 			`${ containerWidth }px`
 		);
 		iframeDocument.documentElement.style.setProperty(
-			'--wp-block-editor-iframe-zoom-out-prev-container-width',
-			`${ prevContainerWidthRef.current }px`
+			'--wp-block-editor-iframe-zoom-out-outer-container-width',
+			`${ Math.max( initialContainerWidth.current, containerWidth ) }px`
 		);
 
-		return () => {
-			iframeDocument.documentElement.classList.remove( 'is-zoomed-out' );
+		// iframeDocument.documentElement.style.setProperty(
+		// 	'--wp-block-editor-iframe-zoom-out-outer-container-width',
+		// 	`${ Math.max( initialContainerWidth.current, containerWidth ) }px`
+		// );
 
+		return () => {
 			iframeDocument.documentElement.style.removeProperty(
 				'--wp-block-editor-iframe-zoom-out-scale'
 			);
@@ -359,7 +400,7 @@ function Iframe( {
 				'--wp-block-editor-iframe-zoom-out-container-width'
 			);
 			iframeDocument.documentElement.style.removeProperty(
-				'--wp-block-editor-iframe-zoom-out-prev-container-width'
+				'--wp-block-editor-iframe-zoom-out-outer-container-width'
 			);
 		};
 	}, [
@@ -460,10 +501,12 @@ function Iframe( {
 					isZoomedOut && 'is-zoomed-out'
 				) }
 				style={ {
-					'--wp-block-editor-iframe-zoom-out-container-width':
-						isZoomedOut && `${ containerWidth }px`,
-					'--wp-block-editor-iframe-zoom-out-prev-container-width':
-						isZoomedOut && `${ prevContainerWidthRef.current }px`,
+					'--wp-block-editor-iframe-zoom-out-outer-container-width':
+						isZoomedOut &&
+						`${ Math.max(
+							initialContainerWidth.current,
+							containerWidth
+						) }px`,
 				} }
 			>
 				{ iframe }

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

@@ -9,9 +9,10 @@
 }
 
 .block-editor-iframe__scale-container.is-zoomed-out {
-	$container-width: var(--wp-block-editor-iframe-zoom-out-container-width, 100vw);
-	$prev-container-width: var(--wp-block-editor-iframe-zoom-out-prev-container-width, 100vw);
-	width: $prev-container-width;
-	// This is to offset the movement of the iframe when we open sidebars
-	margin-left: calc(-1 * (#{$prev-container-width} - #{$container-width}) / 2);
+	$outer-container-width: var(--wp-block-editor-iframe-zoom-out-outer-container-width, 100vw);
+	width: $outer-container-width;
+	// Position the iframe so that it is always aligned with the right side so that
+	// the scrollbar is always visible on the right side
+	position: absolute;
+	right: 0;
 }