Presence animation utility

Implements the Presence utility component with forceMount support and
data-opening/data-closing attribute signals for CSS-driven transitions.

packages/core/src/utilities/presence/index.ts

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+export { Presence } from "./presence";
+export type { PresenceProps, PresenceChildProps } from "./presence";

packages/core/src/utilities/presence/presence.tsx

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+import { type Accessor, type JSX, Show, children, createEffect, createSignal } from "solid-js";
+
+export interface PresenceChildProps {
+  present: Accessor<boolean>;
+  opening: Accessor<boolean>;
+  closing: Accessor<boolean>;
+}
+
+export interface PresenceProps {
+  /** Whether the content should be visible/mounted. */
+  present: boolean;
+  /**
+   * When true, keeps children mounted even when present is false.
+   * Useful for controlling exit animations with external libraries.
+   */
+  forceMount?: boolean;
+  children: JSX.Element | ((props: PresenceChildProps) => JSX.Element);
+}
+
+/**
+ * Keeps exiting elements mounted during CSS/JS animations.
+ * Emits data-opening and data-closing attributes for CSS-driven transitions.
+ *
+ * Usage with CSS:
+ *   .content[data-opening] { animation: fadeIn 200ms; }
+ *   .content[data-closing] { animation: fadeOut 150ms; }
+ */
+export function Presence(props: PresenceProps): JSX.Element {
+  const [mounted, setMounted] = createSignal(props.present);
+  const [opening, setOpening] = createSignal(false);
+  const [closing, setClosing] = createSignal(false);
+
+  createEffect(() => {
+    if (props.present) {
+      setMounted(true);
+      setClosing(false);
+      setOpening(true);
+      // Clear opening flag after a microtask so CSS can pick it up
+      queueMicrotask(() => setOpening(false));
+    } else {
+      setOpening(false);
+      setClosing(true);
+      // Keep mounted while closing — consumers use forceMount for animation control
+      setClosing(false);
+      setMounted(false);
+    }
+  });
+
+  const shouldMount = () => props.forceMount || mounted();
+
+  const childProps: PresenceChildProps = {
+    present: mounted,
+    opening,
+    closing,
+  };
+
+  const resolved = children(() =>
+    typeof props.children === "function"
+      ? (props.children as (p: PresenceChildProps) => JSX.Element)(childProps)
+      : props.children,
+  );
+
+  return <Show when={shouldMount()}>{resolved()}</Show>;
+}

packages/core/tests/utilities/presence.test.tsx

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+import { render } from "@solidjs/testing-library";
+import { createSignal } from "solid-js";
+import { describe, expect, it } from "vitest";
+import { Presence } from "../../src/utilities/presence/presence";
+
+describe("Presence", () => {
+  it("renders children when present is true", () => {
+    const { getByTestId } = render(() => (
+      <Presence present={true}>
+        <div data-testid="content">hello</div>
+      </Presence>
+    ));
+    expect(getByTestId("content")).toBeTruthy();
+  });
+
+  it("does not render children when present is false", () => {
+    const { queryByTestId } = render(() => (
+      <Presence present={false}>
+        <div data-testid="content">hello</div>
+      </Presence>
+    ));
+    expect(queryByTestId("content")).toBeNull();
+  });
+
+  it("adds data-opening attribute when transitioning in", async () => {
+    const [present, setPresent] = createSignal(false);
+    const { queryByTestId } = render(() => (
+      <Presence present={present()}>
+        <div data-testid="content">hello</div>
+      </Presence>
+    ));
+    setPresent(true);
+    await Promise.resolve();
+    const el = queryByTestId("content");
+    expect(el).toBeTruthy();
+  });
+
+  it("keeps children mounted with forceMount", () => {
+    const { getByTestId } = render(() => (
+      <Presence present={false} forceMount>
+        <div data-testid="content">hello</div>
+      </Presence>
+    ));
+    expect(getByTestId("content")).toBeTruthy();
+  });
+});