fix(chat): keep the platform default for Alt-clicked chat links

handleExternalLinkClick treated Alt-click as a plain click and diverted it
into the desktop browser panel. Alt is a link modifier like Cmd/Ctrl/Shift
(Next Link and the sidebar chat links treat it the same), so it now keeps
the platform default.

Author: waleedlatif1

apps/sim/app/workspace/[workspaceId]/home/components/message-content/components/source-link.test.ts

@@ -0,0 +1,53 @@
+/**
+ * @vitest-environment node
+ */
+import type { MouseEvent } from 'react'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+const { mockOpenInBrowserPanel } = vi.hoisted(() => ({ mockOpenInBrowserPanel: vi.fn() }))
+vi.mock('@/lib/browser-agent/open-in-panel', () => ({
+  shouldOpenInBrowserPanel: (href: string | undefined) => Boolean(href),
+  openInBrowserPanel: mockOpenInBrowserPanel,
+}))
+
+import { handleExternalLinkClick } from '@/app/workspace/[workspaceId]/home/components/message-content/components/source-link'
+
+const HREF = 'https://docs.example.com/guide'
+
+function click(modifiers: Partial<MouseEvent<HTMLAnchorElement>> = {}) {
+  const event = {
+    metaKey: false,
+    ctrlKey: false,
+    shiftKey: false,
+    altKey: false,
+    button: 0,
+    preventDefault: vi.fn(),
+    ...modifiers,
+  }
+  handleExternalLinkClick(event as unknown as MouseEvent<HTMLAnchorElement>, HREF)
+  return event
+}
+
+describe('handleExternalLinkClick', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('diverts a plain primary click into the browser panel', () => {
+    const event = click()
+    expect(event.preventDefault).toHaveBeenCalled()
+    expect(mockOpenInBrowserPanel).toHaveBeenCalledWith(HREF)
+  })
+
+  it.each([
+    ['Cmd', { metaKey: true }],
+    ['Ctrl', { ctrlKey: true }],
+    ['Shift', { shiftKey: true }],
+    ['Alt', { altKey: true }],
+    ['middle button', { button: 1 }],
+  ])('keeps the default behavior for a %s click', (_label, modifiers) => {
+    const event = click(modifiers)
+    expect(event.preventDefault).not.toHaveBeenCalled()
+    expect(mockOpenInBrowserPanel).not.toHaveBeenCalled()
+  })
+})

apps/sim/app/workspace/[workspaceId]/home/components/message-content/components/source-link.ts

@@ -28,12 +28,14 @@ export const PROSE_LINK_CLASS =
 
 /**
  * In the desktop app, a plain click diverts into the embedded Sim browser
- * panel; modified clicks (Cmd/Ctrl/Shift/middle) keep the default behavior,
- * which the shell routes to the system browser. In a web browser this is a
- * no-op and the link opens a new tab as usual.
+ * panel; modified clicks (Cmd/Ctrl/Shift/Alt/middle) keep the platform default,
+ * whose new-window opens the shell routes to the system browser. In a web
+ * browser this is a no-op and the link opens a new tab as usual.
  */
 export function handleExternalLinkClick(event: MouseEvent<HTMLAnchorElement>, href: string): void {
-  if (event.metaKey || event.ctrlKey || event.shiftKey || event.button !== 0) return
+  if (event.metaKey || event.ctrlKey || event.shiftKey || event.altKey || event.button !== 0) {
+    return
+  }
   if (!shouldOpenInBrowserPanel(href)) return
   event.preventDefault()
   openInBrowserPanel(href)