Svelte: Fix decorators always running twice

code/e2e-tests/framework-svelte.spec.ts

@@ -50,4 +50,15 @@ test.describe('Svelte', () => {
     const expectedSource = '<ButtonJavaScript primary/>';
     await expect(sourceCode.textContent()).resolves.toContain(expectedSource);
   });
+
+  test('Decorators runs only once', async ({ page }) => {
+    const sbPage = new SbPage(page);
+    const lines: string[] = [];
+    page.on('console', (msg) => {
+      lines.push(msg.text());
+    });
+
+    await sbPage.navigateToStory('stories/renderers/svelte/decorators-runs-once', 'default');
+    expect(lines).toHaveLength(1);
+  });
 });

code/renderers/svelte/template/stories/decorators-runs-once.stories.js

@@ -0,0 +1,16 @@
+import ButtonJavaScript from './views/ButtonJavaScript.svelte';
+
+export default {
+  component: ButtonJavaScript,
+  args: {
+    primary: true,
+  },
+  decorators: [
+    (Story) => {
+      console.log('decorator');
+      return Story();
+    },
+  ],
+};
+
+export const Default = {};

code/renderers/svelte/templates/PreviewRender.svelte

@@ -17,8 +17,25 @@
     on,
   } = storyFn();
 
+	let firstTime = true;
+
+	// the first time we don't want to call storyFn two times so we just return the values
+	// we already have from the previous call. If storyFn changes this function will run
+	// again but this time firstTime will be false
+	function getStoryFnValue(storyFn){
+		if(firstTime){
+			firstTime = false;
+			return {
+				Component,
+				props,
+				on,
+			}
+		}
+		return storyFn();
+	}
+
   // reactive, re-render on storyFn change
-  $: ({ Component, props = {}, on } = storyFn());
+  $: ({ Component, props = {}, on } = getStoryFnValue(storyFn));
 
   const eventsFromArgTypes = Object.fromEntries(
     Object.entries(storyContext.argTypes)