Use fixed positioning on x-ray when inside of fixed containers (#9254)

* Improve highlight/tooltip positioning

* add changeset

* Explain the perf issue

3 files changed, 33 insertions, 6 deletions

diff --git a/.changeset/grumpy-turtles-tickle.md b/.changeset/grumpy-turtles-tickle.md
new file mode 100644
index 000000000..75f8f0eec
--- /dev/null
+++ b/.changeset/grumpy-turtles-tickle.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Improve highlight/tooltip positioning when in fixed positions
diff --git a/packages/astro/src/runtime/client/dev-overlay/plugins/utils/highlight.ts b/packages/astro/src/runtime/client/dev-overlay/plugins/utils/highlight.ts
index e2d74c0ff..565e58a1a 100644
--- a/packages/astro/src/runtime/client/dev-overlay/plugins/utils/highlight.ts
+++ b/packages/astro/src/runtime/client/dev-overlay/plugins/utils/highlight.ts
@@ -15,23 +15,37 @@ export function createHighlight(rect: DOMRect, icon?: Icon) {
 	return highlight;
 }
 
-export function getHighlightZIndex(el: Element) {
+// Figures out the element's z-index and position, based on it's parents.
+export function getElementsPositionInDocument(el: Element) {
 	let highestZIndex = 0;
+	let fixed = false;
 	let current: Element | ParentNode | null = el;
 	while (current instanceof Element) {
-		let zIndex = Number(getComputedStyle(current).zIndex);
+		// This is the expensive part, we are calling getComputedStyle which triggers layout
+		// all the way up the tree. We are only doing so when the app initializes, so the cost is one-time
+		// If perf becomes an issue we'll want to refactor this somehow so that it reads this info in a rAF
+		let style = getComputedStyle(current);
+		let zIndex = Number(style.zIndex);
 		if (!Number.isNaN(zIndex) && zIndex > highestZIndex) {
 			highestZIndex = zIndex;
 		}
+		if(style.position === 'fixed') {
+			fixed = true;
+		}
 		current = current.parentNode;
 	}
-	return highestZIndex + 1;
+	return {
+		zIndex: highestZIndex + 1,
+		fixed,
+	};
 }
 
 export function positionHighlight(highlight: DevOverlayHighlight, rect: DOMRect) {
 	highlight.style.display = 'block';
+	// If the highlight is fixed, don't position based on scroll
+	const scrollY = highlight.style.position === 'fixed' ? 0 : window.scrollY;
 	// Make an highlight that is 10px bigger than the element on all sides
-	highlight.style.top = `${Math.max(rect.top + window.scrollY - 10, 0)}px`;
+	highlight.style.top = `${Math.max(rect.top + scrollY - 10, 0)}px`;
 	highlight.style.left = `${Math.max(rect.left + window.scrollX - 10, 0)}px`;
 	highlight.style.width = `${rect.width + 15}px`;
 	highlight.style.height = `${rect.height + 15}px`;
diff --git a/packages/astro/src/runtime/client/dev-overlay/plugins/xray.ts b/packages/astro/src/runtime/client/dev-overlay/plugins/xray.ts
index 782ede4d2..e76da4de0 100644
--- a/packages/astro/src/runtime/client/dev-overlay/plugins/xray.ts
+++ b/packages/astro/src/runtime/client/dev-overlay/plugins/xray.ts
@@ -3,7 +3,7 @@ import type { DevOverlayHighlight } from '../ui-library/highlight.js';
 import {
 	attachTooltipToHighlight,
 	createHighlight,
-	getHighlightZIndex,
+	getElementsPositionInDocument,
 	positionHighlight,
 } from './utils/highlight.js';
 import { createWindowElement } from './utils/window.js';
@@ -83,8 +83,16 @@ export default {
 				attachTooltipToHighlight(highlight, tooltip, islandElement);
 
 				// Set the z-index to be 1 higher than the greatest z-index in the stack.
-				const zIndex = getHighlightZIndex(islandElement);
+				// And also set the highlight/tooltip as being fixed position if they are inside
+				// a fixed container. We do this so that we don't mistakenly take scroll position
+				// into account when setting their position.
+				// We are only doing both of these things once, as they are relatively expensive
+				// to calculate, and are unlikely to change. If that turns out to be wrong, reconsider this.
+				const { zIndex, fixed } = getElementsPositionInDocument(islandElement);
 				tooltip.style.zIndex = highlight.style.zIndex = zIndex + '';
+				if(fixed) {
+					tooltip.style.position = highlight.style.position = 'fixed';
+				}
 
 				canvas.append(highlight);
 				islandsOverlays.push({ highlightElement: highlight, island: islandElement });