Regression: server action is not working from v2.43.0 if the submit button is a custom element when use:enhance is used

Describe the bug

Server actions are not triggered if the submit button is a custom element. The error is occurring since v2.43.0.

# simple  server action +page.server.ts
import { fail } from '@sveltejs/kit';

export const actions = {
	default: async () => {
		try {
			console.log('Hello from server action');

			return { message: "success" };
		} catch (error) {
			console.log(error);
			return fail(500, { message: "error in server side" });
		}
	},
};

#+page.svelte with custom element definition
<script lang="ts">
	import { onMount } from 'svelte';
	import { applyAction, enhance } from '$app/forms';
	import { goto } from '$app/navigation';

	let mounted = $state(false);

	onMount(async () => {
		customElements.define('custom-button', class Button extends HTMLElement {
			buttonBase: HTMLButtonElement;
			constructor() {
				super();
				this.buttonBase = document.createElement("button");
				this.buttonBase.style = "cursor: pointer";
				this.buttonBase.append(document.createElement("slot"));

				const shadow = this.attachShadow({ mode: "open", delegatesFocus: true });
				shadow.append(this.buttonBase);

				this.handleFormControl = () => {
					const form = this.closest("form");
					if (form && this.type) {
						if (this.type === "reset") return form.reset();
						else if (this.type === "submit") {
							const event = new SubmitEvent("submit", {
								submitter: this,
								cancelable: true
							});
							form.reportValidity();
							form.dispatchEvent(event);
							setTimeout(() => {
								if (event.defaultPrevented) return;
								if (form.checkValidity()) form.submit();
							});
						}
					}
				}
			}
			connectedCallback() {
				this.buttonBase.addEventListener("click", this.handleFormControl);
			}
			disconnectedCallback() {
				this.buttonBase.removeEventListener("click", this.handleFormControl);
			}
			get type() {
				return this.getAttribute('type');
			}
			set type(val) {
				if (val) {
					this.setAttribute('type', val);
				} else {
					this.removeAttribute('type');
				}
			}
		});
		mounted = true;
	});
</script>

<h1>Welcome to SvelteKit</h1>
<p>Visit <a href="https://svelte.dev/docs/kit">svelte.dev/docs/kit</a> to read the documentation</p>

{#if mounted}
	<form
		novalidate={true}
		method="POST"
		use:enhance={() => {
			return async ({ result }) => {
				// `result` is an `ActionResult` object
				if (result.type === 'redirect') {
					goto(result.location);
				} else {
					await applyAction(result);
				}
			};
		}}
	>
		<custom-button type="submit">
			Submit
		</custom-button>
	</form>
{/if}

Reproduction

github repo
stackblitz
It was working with version: 2.42.0

Logs

client:733 [vite] connecting...
client:827 [vite] connected.
forms.js:139  Uncaught (in promise) TypeError: Failed to construct 'FormData': The specified element is not a submit button.
    at HTMLFormElement.handle_submit (forms.js:139:21)
    at Button.handleFormControl (+page.svelte:29:13)
handle_submit @ forms.js:139
Button.handleFormControl @ +page.svelte:29

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (8) x64 11th Gen Intel(R) Core(TM) i5-1145G7 @ 2.60GHz
    Memory: 2.93 GB / 15.73 GB
  Binaries:
    Node: 22.14.0 - C:\Users\supab1\nodejs\node.EXE
    npm: 11.4.1 - C:\Users\supab1\nodejs\npm.CMD
    pnpm: 10.17.1 - C:\Users\supab1\AppData\Local\pnpm\pnpm.EXE
    bun: 1.2.21 - C:\Users\supab1\.bun\bin\bun.EXE
    Deno: 2.5.2 - C:\Users\supab1\.deno\bin\deno.EXE
  Browsers:
    Edge: Chromium (140.0.3485.54)
    Firefox: 133.0.3 - C:\Program Files\Mozilla Firefox\firefox.exe
    Internet Explorer: 11.0.19041.5794
  npmPackages:
    @sveltejs/adapter-node: ^5.3.2 => 5.3.3
    @sveltejs/kit: ^2.46.4 => 2.46.4
    @sveltejs/vite-plugin-svelte: ^6.2.0 => 6.2.1
    svelte: ^5.39.5 => 5.39.11
    vite: ^7.1.7 => 7.1.9

Severity

blocking an upgrade

Additional Information

This may be the button element is not having type=submit but this used to work earlier

Start with forms.js around handle_submit at line 139 and reproduce the provided +page.svelte example using the linked repository or StackBlitz. Compare the behavior with version 2.42.0, then verify that a custom submit element used with use:enhance triggers the server action without the FormData TypeError.