feat: better flip animations (#13317)

* fix: flip not accounting for scaled lengths

* zoom is uniform, one calculation will do

* stash width/height multipliers

* use var

* scale-first appears to work better?

* more robust zoom calculation

* changeset

---------

Co-authored-by: Etai Nadler <[email protected]>

Author: Rich-Harris

.changeset/cold-socks-learn.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+feat: account for `zoom` when calculating animation transforms

packages/svelte/src/animate/index.js

@@ -12,22 +12,48 @@ import { cubicOut } from '../easing/index.js';
  * @returns {AnimationConfig}
  */
 export function flip(node, { from, to }, params = {}) {
-	const style = getComputedStyle(node);
-	const transform = style.transform === 'none' ? '' : style.transform;
-	const [ox, oy] = style.transformOrigin.split(' ').map(parseFloat);
-	const dx = from.left + (from.width * ox) / to.width - (to.left + ox);
-	const dy = from.top + (from.height * oy) / to.height - (to.top + oy);
-	const { delay = 0, duration = (d) => Math.sqrt(d) * 120, easing = cubicOut } = params;
+	var style = getComputedStyle(node);
+	var zoom = get_zoom(node); // https://drafts.csswg.org/css-viewport/#effective-zoom
+
+	var transform = style.transform === 'none' ? '' : style.transform;
+	var [ox, oy] = style.transformOrigin.split(' ').map(parseFloat);
+	var dsx = from.width / to.width;
+	var dsy = from.height / to.height;
+
+	var dx = (from.left + dsx * ox - (to.left + ox)) / zoom;
+	var dy = (from.top + dsy * oy - (to.top + oy)) / zoom;
+	var { delay = 0, duration = (d) => Math.sqrt(d) * 120, easing = cubicOut } = params;
+
 	return {
 		delay,
 		duration: typeof duration === 'function' ? duration(Math.sqrt(dx * dx + dy * dy)) : duration,
 		easing,
 		css: (t, u) => {
-			const x = u * dx;
-			const y = u * dy;
-			const sx = t + (u * from.width) / to.width;
-			const sy = t + (u * from.height) / to.height;
-			return `transform: ${transform} translate(${x}px, ${y}px) scale(${sx}, ${sy});`;
+			var x = u * dx;
+			var y = u * dy;
+			var sx = t + u * dsx;
+			var sy = t + u * dsy;
+			return `transform: ${transform} scale(${sx}, ${sy}) translate(${x}px, ${y}px);`;
 		}
 	};
 }
+
+/**
+ * @param {Element} element
+ */
+function get_zoom(element) {
+	if ('currentCSSZoom' in element) {
+		return /** @type {number} */ (element.currentCSSZoom);
+	}
+
+	/** @type {Element | null} */
+	var current = element;
+	var zoom = 1;
+
+	while (current !== null) {
+		zoom *= +getComputedStyle(current).zoom;
+		current = /** @type {Element | null} */ (current.parentNode);
+	}
+
+	return zoom;
+}