Experiment: add moveBefore rather than insertBefor when available

src/constants.js

@@ -33,3 +33,6 @@ export const EMPTY_OBJ = /** @type {any} */ ({});
 export const EMPTY_ARR = [];
 
 export const MATHML_TOKEN_ELEMENTS = /(mi|mn|mo|ms$|mte|msp)/;
+
+export const HAS_MOVE_BEFORE_SUPPORT =
+	typeof window !== 'undefined' && 'moveBefore' in Element.prototype;

src/diff/children.js

@@ -6,7 +6,8 @@ import {
 	INSERT_VNODE,
 	MATCHED,
 	UNDEFINED,
-	NULL
+	NULL,
+	HAS_MOVE_BEFORE_SUPPORT
 } from '../constants';
 import { isArray } from '../util';
 import { getDomSibling } from '../component';
@@ -127,7 +128,13 @@ export function diffChildren(
 
 		let shouldPlace = childVNode._flags & INSERT_VNODE;
 		if (shouldPlace || oldVNode._children === childVNode._children) {
-			oldDom = insert(childVNode, oldDom, parentDom, shouldPlace);
+			oldDom = insert(
+				childVNode,
+				oldDom,
+				parentDom,
+				shouldPlace,
+				oldVNode == NULL || oldVNode._original == NULL
+			);
 
 			// When a matched VNode is physically moved via INSERT_VNODE, its old
 			// _dom pointer becomes a stale positional reference. Clear it so that
@@ -344,9 +351,10 @@ function constructNewChildrenArray(
  * @param {PreactElement} oldDom
  * @param {PreactElement} parentDom
  * @param {number} shouldPlace
+ * @param {boolean} isMounting
  * @returns {PreactElement}
  */
-function insert(parentVNode, oldDom, parentDom, shouldPlace) {
+function insert(parentVNode, oldDom, parentDom, shouldPlace, isMounting) {
 	// Note: VNodes in nested suspended trees may be missing _children.
 	if (typeof parentVNode.type == 'function') {
 		let children = parentVNode._children;
@@ -357,7 +365,7 @@ function insert(parentVNode, oldDom, parentDom, shouldPlace) {
 				// children's _parent pointer to point to the newVNode (parentVNode
 				// here).
 				children[i]._parent = parentVNode;
-				oldDom = insert(children[i], oldDom, parentDom, shouldPlace);
+				oldDom = insert(children[i], oldDom, parentDom, shouldPlace, false);
 			}
 		}
 
@@ -367,7 +375,13 @@ function insert(parentVNode, oldDom, parentDom, shouldPlace) {
 			if (oldDom && parentVNode.type && !oldDom.parentNode) {
 				oldDom = getDomSibling(parentVNode);
 			}
-			parentDom.insertBefore(parentVNode._dom, oldDom || NULL);
+
+			if (HAS_MOVE_BEFORE_SUPPORT && !isMounting) {
+				// @ts-expect-error This isn't added to TypeScript lib.d.ts yet
+				parentDom.moveBefore(parentVNode._dom, oldDom);
+			} else {
+				parentDom.insertBefore(parentVNode._dom, oldDom || NULL);
+			}
 		}
 		oldDom = parentVNode._dom;
 	}

test/_util/logCall.js

@@ -39,6 +39,14 @@ export function logCall(obj, method) {
 				}
 				break;
 			}
+			case 'moveBefore': {
+				if (args[1] === null && args.length === 2) {
+					operation = `${serialize(this)}.appendChild(${serialize(args[0])})`;
+				} else {
+					operation = `${serialize(this)}.insertBefore(${c})`;
+				}
+				break;
+			}
 			default: {
 				operation = `${serialize(this)}.${String(method)}(${c})`;
 				break;

test/browser/fragments.test.jsx

@@ -38,9 +38,12 @@ describe('Fragment', () => {
 	let resetAppendChild;
 	let resetRemove;
 	let resetRemoveText;
+	let resetMoveBefore;
 
 	beforeAll(() => {
 		resetInsertBefore = logCall(Element.prototype, 'insertBefore');
+		// @ts-expect-error
+		resetMoveBefore = logCall(Element.prototype, 'moveBefore');
 		resetAppendChild = logCall(Element.prototype, 'appendChild');
 		resetRemove = logCall(Element.prototype, 'remove');
 		resetRemoveText = logCall(Text.prototype, 'remove');
@@ -57,6 +60,7 @@ describe('Fragment', () => {
 	});
 
 	afterAll(() => {
+		resetMoveBefore();
 		resetInsertBefore();
 		resetAppendChild();
 		resetRemove();

test/browser/keys.test.jsx

@@ -71,13 +71,15 @@ describe('keys', () => {
 		values.splice(to, 0, value);
 	}
 
+	let resetMoveBefore;
 	let resetAppendChild;
 	let resetInsertBefore;
 	let resetRemoveChild;
 	let resetRemove;
 	let resetRemoveText;
 
 	beforeAll(() => {
+		resetMoveBefore = logCall(Element.prototype, 'moveBefore');
 		resetAppendChild = logCall(Element.prototype, 'appendChild');
 		resetInsertBefore = logCall(Element.prototype, 'insertBefore');
 		resetRemoveChild = logCall(Element.prototype, 'removeChild');
@@ -91,6 +93,7 @@ describe('keys', () => {
 		resetRemoveChild();
 		resetRemove();
 		resetRemoveText();
+		resetMoveBefore();
 	});
 
 	beforeEach(() => {

test/browser/lifecycles/shouldComponentUpdate.test.jsx

@@ -15,10 +15,13 @@ describe('Lifecycle methods', () => {
 	// 	expect(getLog()).to.deep.equal(expectedOperations, message);
 	// }
 	let resetInsertBefore;
+	let resetMoveBefore;
 	let resetRemoveChild;
 	let resetRemove;
 
 	beforeAll(() => {
+		// @ts-expect-error
+		resetMoveBefore = logCall(Element.prototype, 'moveBefore');
 		resetInsertBefore = logCall(Element.prototype, 'insertBefore');
 		resetRemoveChild = logCall(Element.prototype, 'appendChild');
 		resetRemove = logCall(Element.prototype, 'removeChild');
@@ -28,6 +31,7 @@ describe('Lifecycle methods', () => {
 		resetInsertBefore();
 		resetRemoveChild();
 		resetRemove();
+		resetMoveBefore();
 	});
 
 	beforeEach(() => {

test/browser/render.test.jsx

@@ -30,6 +30,7 @@ describe('render()', () => {
 	let resetInsertBefore;
 	let resetRemoveText;
 	let resetRemove;
+	let resetMoveBefore;
 
 	beforeEach(() => {
 		scratch = setupScratch();
@@ -43,12 +44,15 @@ describe('render()', () => {
 	beforeAll(() => {
 		resetAppendChild = logCall(Element.prototype, 'appendChild');
 		resetInsertBefore = logCall(Element.prototype, 'insertBefore');
+		// @ts-expect-error
+		resetMoveBefore = logCall(Element.prototype, 'moveBefore');
 		resetRemoveText = logCall(Text.prototype, 'remove');
 		resetRemove = logCall(Element.prototype, 'remove');
 	});
 
 	afterAll(() => {
 		resetAppendChild();
+		resetMoveBefore();
 		resetInsertBefore();
 		resetRemoveText();
 		resetRemove();