Fixes to issues with Dialog, FocusTrap, and StackContextProvider

* StackContextProvider had some reactivity that didn't make sense and would cause an infinite loop if it ever triggered.
* FocusTrap and Dialog did not work together properly all of the time. A top-level <Dialog> with initial state of closed would not have the FocusTrap triggered correctly when it was opened. Needed to make it reactive to changes in `containers`

src/lib/components/dialog/Dialog.svelte

@@ -253,10 +253,11 @@
   onUpdate={(message, element) => {
     return match(message, {
       [StackMessage.Add]() {
-        containers.add(element);
+        containers = new Set([...containers, element]);
       },
       [StackMessage.Remove]() {
         containers.delete(element);
+        containers = new Set([...containers]);
       },
     });
   }}

src/lib/components/focus-trap/FocusTrap.svelte

@@ -7,7 +7,7 @@
     FocusResult,
   } from "$lib/utils/focus-management";
   import { contains } from "$lib/internal/dom-containers";
-  import { afterUpdate, onMount, onDestroy, tick } from "svelte";
+  import { onMount, onDestroy, tick } from "svelte";
 
   export let containers: Set<HTMLElement>;
   export let enabled: boolean = true;
@@ -74,7 +74,7 @@
   // Handle initial focus
   onMount(handleFocus);
 
-  afterUpdate(() => (enabled ? handleFocus() : restore()));
+  $: enabled && containers ? handleFocus() : restore();
 
   // When this component is being destroyed, focusElement is called
   // before handleWindowFocus is removed, so in the svelte port we add this

src/lib/internal/StackContextProvider.svelte

@@ -9,24 +9,21 @@
 
 <script lang="ts">
   import { getContext, onDestroy, setContext } from "svelte";
-  import { Readable, writable, Writable } from "svelte/store";
   type OnUpdate = (message: StackMessage, element: HTMLElement) => void;
 
   export let onUpdate: OnUpdate | undefined;
   export let element: HTMLElement | null;
 
-  let parentUpdateStore: Readable<OnUpdate> | undefined =
+  function notify(...args: Parameters<OnUpdate>) {
-    getContext(STACK_CONTEXT_NAME);
-  let notifyStore: Writable<OnUpdate> = writable(() => {});
-  setContext(STACK_CONTEXT_NAME, notifyStore);
-
-  $: notifyStore.set((...args: Parameters<OnUpdate>) => {
     // Notify our layer
     onUpdate?.(...args);
 
     // Notify the parent
-    $parentUpdateStore?.(...args);
+    parentUpdate?.(...args);
-  });
+  }
+
+  let parentUpdate: OnUpdate | undefined = getContext(STACK_CONTEXT_NAME);
+  setContext(STACK_CONTEXT_NAME, notify);
 
   $: _cleanup = (() => {
     if (_cleanup) {
@@ -34,8 +31,8 @@
     }
     if (!element) return null;
     let savedElement = element;
-    $notifyStore(StackMessage.Add, savedElement);
+    notify(StackMessage.Add, savedElement);
-    return () => $notifyStore(StackMessage.Remove, savedElement);
+    return () => notify(StackMessage.Remove, savedElement);
   })();
 
   onDestroy(() => {