seems to work now

apps/dev/sveltekit/src/lib/SignInButton.svelte

@@ -1,12 +1,11 @@
 <script lang="ts">
   export let provider: any
+  export let callbackUrl: string
 </script>
 
 <form action={provider.signinUrl} method="POST">
-  {#if provider.callbackUrl}
-    <input type="hidden" name="callbackUrl" value={provider.callbackUrl} />
-  {/if}
-  <button type="submit" class="button">
+  <input type="hidden" name="callbackUrl" value={callbackUrl} />
+  <button type="submit">
     <slot>Sign in with {provider.name}</slot>
   </button>
 </form>

apps/dev/sveltekit/src/routes/+layout.server.ts

@@ -4,5 +4,6 @@ export const load: LayoutServerLoad = async (event) => {
   return {
     session: await event.locals.getSession(),
     providers: await event.locals.getProviders(),
+    callbackUrl: new URL(event.request.url).host,
   }
 }

apps/dev/sveltekit/src/routes/js-disabled/+page.svelte

@@ -19,11 +19,11 @@
     >
   </span>
   <form action="/auth/signout" method="POST">
-    <button id="submitButton" type="submit">Sign out</button>
+    <button type="submit">Sign out</button>
   </form>
 {:else}
   <span class="notSignedInText">You are not signed in</span>
   {#each Object.values($page.data.providers) as provider}
-    <SignInButton {provider} />
+    <SignInButton {provider} callbackUrl={$page.data.callbackUrl} />
   {/each}
 {/if}