Support style as a function

Seems useful to add.

Also improve the prop typing of <Render>, marginally, though as these types do not carry over to end users yet, it's just cosmetic for now.

src/lib/components/description/Description.svelte

@@ -22,7 +22,7 @@
 
   onMount(() => $contextStore?.register(id));
 
-  $: slotProps = $contextStore!.slotProps;
+  $: slotProps = $contextStore!.slotProps || {};
 </script>
 
 <Render

src/lib/components/popover/PopoverGroup.svelte

@@ -71,6 +71,7 @@
   {...$$restProps}
   {as}
   use={[...use, forwardEvents]}
+  slotProps={{}}
   name={"PopoverGroup"}
   bind:el={groupRef}
 >

src/lib/components/radio-group/RadioGroup.svelte

@@ -208,6 +208,7 @@
       {...{ ...$$restProps, ...propsWeControl }}
       {as}
       use={[...use, forwardEvents]}
+      slotProps={{}}
       name={"RadioGroup"}
       bind:el={radioGroupRef}
       aria-labelledby={labelledby}

src/lib/components/switch/SwitchGroup.svelte

@@ -42,6 +42,7 @@
   {...$$restProps}
   {as}
   use={[...use, forwardEvents]}
+  slotProps={{}}
   name={"SwitchGroup"}
 >
   <DescriptionProvider name="SwitchDescription">

src/lib/components/transitions/TransitionChild.svelte

@@ -195,6 +195,7 @@
   {...$$restProps}
   {as}
   use={[...use, forwardEvents]}
+  slotProps={{}}
   name={"TransitionChild"}
   bind:el={container}
   class={classes}

src/lib/utils/Render.svelte

@@ -34,16 +34,27 @@
   import { forwardEventsBuilder } from "$lib/internal/forwardEventsBuilder";
   const forwardEvents = forwardEventsBuilder(get_current_component());
 
+  type TSlotProps = $$Generic<{}>;
+
   export let name: string;
   export let as: SvelteComponent | SupportedElement;
+  export let slotProps: TSlotProps;
+
   export let el: HTMLElement | null = null;
   export let use: ActionArray = [];
-  export let slotProps: unknown = {};
   export let visible = true;
   export let features: Features = Features.None;
   // The static and unmount props are only used in conjunction with the render strategies
   export let unmount = true;
 
+  let classProp: ((props: TSlotProps) => string) | string | undefined =
+    undefined;
+  export { classProp as class };
+
+  // This is not in upstream Headless UI, but we might as well add it here
+  export let style: ((props: TSlotProps) => string) | string | undefined =
+    undefined;
+
   if (!as) {
     throw new Error(`<${name}> did not provide an \`as\` value to <Render>`);
   }
@@ -55,7 +66,9 @@
     );
   }
 
-  $: classStyle = $$props.class;
+  $: computedClass =
+    typeof classProp === "function" ? classProp(slotProps) : classProp;
+  $: computedStyle = typeof style === "function" ? style(slotProps) : style;
 
   $: show =
     visible ||
@@ -74,10 +87,9 @@
     bind:el
     use={[...use, forwardEvents]}
     {...$$restProps}
-    class={typeof classStyle === "function"
-      ? classStyle(slotProps)
-      : classStyle}
-    style={hidden ? "display: none" : $$props.style}
+    class={computedClass}
+    style={`${computedStyle ?? ""}${hidden ? " display: none" : ""}` ||
+      undefined}
     hidden={hidden || undefined}
   >
     <slot />

src/lib/utils/render.test.ts

@@ -0,0 +1,52 @@
+import Render from "$lib/utils/Render.svelte";
+// import svelte from "svelte-inline-compile";
+import { getByTestId, render } from "@testing-library/svelte";
+
+it("should be possible to use class as a string", () => {
+  let { container } = render(Render, {
+    as: "div",
+    name: "test",
+    slotProps: {},
+    class: "test-class",
+    "data-testid": "test-id",
+  });
+  let element = getByTestId(container, "test-id");
+  expect(element).toHaveClass("test-class");
+});
+
+it("should be possible to use class as a function", () => {
+  render(Render, {
+    as: "div",
+    name: "test",
+    slotProps: { foo: "bar" },
+    class: JSON.stringify,
+    id: "test-id",
+  });
+  expect("" + document.querySelector("#test-id")?.classList).toEqual(
+    JSON.stringify({ foo: "bar" })
+  );
+});
+
+it("should be possible to use style as a string", () => {
+  let { container } = render(Render, {
+    as: "div",
+    name: "test",
+    slotProps: {},
+    style: "background-color: green",
+    "data-testid": "test-id",
+  });
+  let element = getByTestId(container, "test-id");
+  expect(element).toHaveStyle("background-color: green");
+});
+
+it("should be possible to use style as a function", () => {
+  let { container } = render(Render, {
+    as: "div",
+    name: "test",
+    slotProps: { enabled: true },
+    style: ({ enabled }: { enabled: boolean }) => enabled ? "background-color: red" : "background-color: green",
+    "data-testid": "test-id",
+  });
+  let element = getByTestId(container, "test-id");
+  expect(element).toHaveStyle("background-color: red");
+});