refactor(core)!: tighten constrained jsx boundary

packages/utils/src/dom/types.ts

@@ -1,3 +1,5 @@
+import type { StringWithSuggestions } from '../types';
+
 // Method syntax is required here for TypeScript's class inheritance checking.
 // Using property syntax (e.g., `connectedCallback?: () => void`) causes TS2425
 // when a class extends a generic mixin that defines lifecycle callbacks.
@@ -18,7 +20,7 @@ export type QueriedElement<S extends string, E extends Element> = S extends keyo
   ? HTMLElementTagNameMap[S]
   : E;
 
-export type EventType<Events> = (keyof Events & string) | (string & {});
+export type EventType<Events> = StringWithSuggestions<keyof Events & string>;
 
 export type EventListenerFor<Events, K> =
   | ((event: K extends keyof Events ? Events[K] : Event) => void)

packages/utils/src/types/tests/types.test.ts

@@ -0,0 +1,12 @@
+import { describe, expectTypeOf, it } from 'vitest';
+import type { StringWithSuggestions } from '../types';
+
+describe('StringWithSuggestions', () => {
+  it('accepts arbitrary strings without widening literal unions', () => {
+    type Action = StringWithSuggestions<'play' | 'pause'>;
+
+    expectTypeOf<Action>().toMatchTypeOf<string>();
+    expectTypeOf<string>().toMatchTypeOf<Action>();
+    expectTypeOf<Extract<Action, 'play'>>().toEqualTypeOf<'play'>();
+  });
+});

packages/utils/src/types/types.ts

@@ -15,6 +15,8 @@ export type MixinReturn<Base extends AnyConstructor<any>, Props> = Constructor<I
 
 export type Falsy<T> = T | false | null | undefined;
 
+export type StringWithSuggestions<Value extends string> = Value | (string & {});
+
 export type EnsureFunction<T> = T extends (...args: any[]) => any ? T : never;
 
 export type Simplify<T> = { [KeyType in keyof T]: T[KeyType] } & {};