Improve layout rendering on interaction (e.g. typing)

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

@@ -14,6 +14,14 @@ $block-inserter-tabs-height: 44px;
 	@include break-medium {
 		position: relative;
 	}
+
+	&__content {
+		// This improves render performance when scrolling.
+		// It promotes this element to a dedicated "layer" in the browser. The rendering
+		// is then assisted by the GPU, which improves performance.
+		// See: https://aerotwist.com/blog/on-translate3d-and-layer-creation-hacks/
+		transform: translate3d(0, 0, 0);
+	}
 }
 
 .block-editor-inserter__popover .block-editor-inserter__menu {

packages/edit-post/src/components/layout/style.scss

@@ -147,3 +147,15 @@
 		height: 100%;
 	}
 }
+
+.edit-post-sidebar {
+	// This improves render performance when scrolling.
+	// It promotes this element to a dedicated "layer" in the browser. The rendering
+	// is then assisted by the GPU, which improves performance.
+	// See: https://aerotwist.com/blog/on-translate3d-and-layer-creation-hacks/
+	transform: translate3d(0, 0, 0);
+
+	> .components-panel {
+		transform: translate3d(0, 0, 0);
+	}
+}

packages/interface/package.json

@@ -29,6 +29,7 @@
 	"dependencies": {
 		"@babel/runtime": "^7.9.2",
 		"@wordpress/components": "file:../components",
+		"@wordpress/compose": "file:../compose",
 		"@wordpress/data": "file:../data",
 		"@wordpress/element": "file:../element",
 		"@wordpress/i18n": "file:../i18n",

packages/interface/src/components/interface-skeleton/index.js

@@ -6,10 +6,15 @@ import classnames from 'classnames';
 /**
  * WordPress dependencies
  */
-import { useEffect } from '@wordpress/element';
+import { useEffect, useRef } from '@wordpress/element';
 import { navigateRegions } from '@wordpress/components';
 import { __ } from '@wordpress/i18n';
 
+/**
+ * Internal dependencies
+ */
+import { useObserveInterfaceHeight } from './utils';
+
 function useHTMLClass( className ) {
 	useEffect( () => {
 		const element =
@@ -36,6 +41,11 @@ function InterfaceSkeleton( {
 } ) {
 	useHTMLClass( 'interface-interface-skeleton__html-container' );
 
+	const bodyNodeRef = useRef();
+	const { resizeListener } = useObserveInterfaceHeight( {
+		ref: bodyNodeRef,
+	} );
+
 	const defaultLabels = {
 		/* translators: accessibility text for the top bar landmark region. */
 		header: __( 'Header' ),
@@ -60,6 +70,7 @@ function InterfaceSkeleton( {
 				'interface-interface-skeleton'
 			) }
 		>
+			{ resizeListener }
 			{ !! header && (
 				<div
 					className="interface-interface-skeleton__header"
@@ -70,7 +81,10 @@ function InterfaceSkeleton( {
 					{ header }
 				</div>
 			) }
-			<div className="interface-interface-skeleton__body">
+			<div
+				className="interface-interface-skeleton__body"
+				ref={ bodyNodeRef }
+			>
 				{ !! leftSidebar && (
 					<div
 						className="interface-interface-skeleton__left-sidebar"

packages/interface/src/components/interface-skeleton/style.scss

@@ -57,6 +57,27 @@ html.interface-interface-skeleton__html-container {
 	overscroll-behavior-y: none;
 }
 
+.interface-interface-skeleton__content,
+.interface-interface-skeleton__left-sidebar,
+.interface-interface-skeleton__sidebar {
+	// This improves render performance when interacting with the DOM.
+	// The Interface scrollable elements use adaptive CSS properties to calculate
+	// height on initial render. However, anytime interactions happen
+	// (e.g. typing), the browser needs to recalculate layout measurements
+	// in order to adjust scroll height.
+	// The --wp-editor--body-height CSS variable is calculated by JavaScript.
+	// It provides a fixed (px) value, which reduces the browser's need to
+	// recalculate height values on interaction. The height value is updated
+	// on resize-based events.
+	height: var(--wp-editor--body-height, 100%);
+
+	// This improves render performance when scrolling.
+	// It promotes this element to a dedicated "layer" in the browser. The rendering
+	// is then assisted by the GPU, which improves performance.
+	// See: https://aerotwist.com/blog/on-translate3d-and-layer-creation-hacks/
+	transform: translate3d(0, 0, 0);
+}
+
 .interface-interface-skeleton__content {
 	flex-grow: 1;
 

packages/interface/src/components/interface-skeleton/utils.js

@@ -0,0 +1,40 @@
+/**
+ * WordPress dependencies
+ */
+import { useEffect } from '@wordpress/element';
+import { useResizeObserver } from '@wordpress/compose';
+
+const SKELETON_BODY_HEIGHT_CSS_PROP = '--wp-editor--body-height';
+
+export function useObserveInterfaceHeight( { ref } ) {
+	const [ resizeListener, sizes ] = useResizeObserver();
+
+	/*
+	 * This calculates and sets a body height for the inner
+	 * content, sidebar, and complementary areas. This improves performance,
+	 * as we can avoid having the browser (re)calculate layout values on
+	 * interaction (e.g. typing).
+	 */
+	const setBodyHeightValue = () => {
+		const bodyHeight = ref.current?.clientHeight;
+		if ( bodyHeight ) {
+			const nextHeight = window.innerHeight - bodyHeight;
+			// We're using calc with a 100vh offset improve responsiveness
+			// when resizing the browser's height. Using a fixed pixel value
+			// directly may result in double scrollbar flickering.
+			const nextHeightValue = `calc(100vh - ${ nextHeight }px)`;
+
+			// We'll set the value as a custom CSS variable.
+			// The inner content, sidebar, and complementary areas are
+			// "subscribed" to this value using CSS.
+			document.documentElement.style.setProperty(
+				SKELETON_BODY_HEIGHT_CSS_PROP,
+				nextHeightValue
+			);
+		}
+	};
+
+	useEffect( setBodyHeightValue, [ sizes.height ] );
+
+	return { resizeListener };
+}