feat: Add "open in new tab" link to history entries

Each history entry now has a third action: a real anchor (rendered via
the Button's href, target="_blank") linking to the editor with that
entry's serialized state. Being a normal link, users can also copy it or
open it in a new tab via the context menu.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/components/History/History.svelte

@@ -2,6 +2,7 @@
   import Card from '$lib/components/Card/Card.svelte';
   import type { HistoryEntry, HistoryType, State, Tab } from '$lib/types';
   import { notify, prompt } from '$lib/util/notify';
+  import { serializeState } from '$lib/util/serde';
   import { inputStateStore } from '$lib/util/state';
   import { logEvent } from '$lib/util/stats';
   import dayjs from 'dayjs';
@@ -14,6 +15,7 @@
   import UploadIcon from '~icons/material-symbols/upload-rounded';
   import HistoryIcon from '~icons/mdi/clock-outline';
   import GitAltIcon from '~icons/mdi/git';
+  import OpenInNewIcon from '~icons/material-symbols/open-in-new-rounded';
   import { Button } from '../ui/button';
   import { Separator } from '../ui/separator';
   import {
@@ -99,6 +101,10 @@
   const restoreHistoryItem = (state: State): void => {
     inputStateStore.set({ ...state, updateDiagram: true });
   };
+
+  // Absolute editor URL for an entry, so the link can be opened in a new tab or copied.
+  const entryUrl = (state: State): string =>
+    `${window.location.origin}${window.location.pathname}#${serializeState(state)}`;
 </script>
 
 <Card onselect={tabSelectHandler} isOpen isClosable={false} {tabs} activeTabID={historyState.mode}>
@@ -160,6 +166,15 @@
               <span class="text-sm whitespace-nowrap text-primary-foreground/50">
                 {dayjs(time).fromNow()}
               </span>
+              <Button
+                href={entryUrl(state)}
+                target="_blank"
+                rel="noopener"
+                size="icon"
+                variant="ghost"
+                title="Open in new tab">
+                <OpenInNewIcon />
+              </Button>
               <Button
                 size="icon"
                 variant="ghost"

tests/history.spec.ts

@@ -67,6 +67,25 @@ test.describe('History', () => {
     await expect(page.locator('#view')).toContainText('NewYear');
   });
 
+  test('each entry has a copyable link that opens it in a new tab', async ({ page }) => {
+    await page.evaluate(
+      (manual) => localStorage.setItem('manualHistoryStore', manual),
+      JSON.stringify(manualHistory)
+    );
+    await page.reload();
+    await openHistory(page);
+
+    // It is a real link (so it can be copied / opened in a new tab), not a button.
+    const link = page.getByRole('link', { name: 'Open in new tab' }).first();
+    await expect(link).toHaveAttribute('target', '_blank');
+    const href = await link.getAttribute('href');
+    expect(href).toContain('/edit#pako:');
+
+    // Following it loads that entry's diagram.
+    await page.goto(href ?? '');
+    await expect(page.locator('#view')).toContainText('Halloween');
+  });
+
   test('keeps the active tab highlighted when switching modes', async ({ page }) => {
     await openHistory(page);
     const saved = page.getByRole('tab', { name: 'Saved' });