fix: repair href attribute mismatches

The reasoning in #9662 turns out to be false: While it's true that the href would resolve to the same URL on the initial page, once you're doing a client-side navigation to a URL at a different depth, the relative path would now point to the wrong location. Therefore we need to repair href hydration mismatches

fixes sveltejs/kit#12254

Author: dummdidumm

.changeset/two-cats-approve.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: repair `href` attribute mismatches

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -89,7 +89,12 @@ export function set_attribute(element, attribute, value, skip_warning) {
 	if (hydrating) {
 		attributes[attribute] = element.getAttribute(attribute);
 
-		if (attribute === 'src' || attribute === 'href' || attribute === 'srcset') {
+		// Don't include href: SvelteKit will by default render relative URLs but on the client they
+		// need to be hydrated to root-relative URLs in case you have a base path set. Else they
+		// could point to the wrong location once you click a link in a layout (if the links
+		// in the layout don't update, which they won't if they don't depend on page data, then they
+		// could be relative to a different URL and therefore wrong).
+		if (attribute === 'src' || attribute === 'srcset') {
 			if (!skip_warning) {
 				check_src_in_dev_hydration(element, attribute, value);
 			}