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 +89,11 @@ 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: When using a base path, SvelteKit will by default render relative URLs on the server
+		// and use root-relative links on the client. While 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 mismatches.
+		if (attribute === 'src' || attribute === 'srcset') {
 			if (!skip_warning) {
 				check_src_in_dev_hydration(element, attribute, value);
 			}