Fix Cmd/Ctrl-click on sub-page links not opening new tab

[falkoschindler]

Motivation

When using ui.link inside a ui.sub_pages context, Cmd-click (macOS) or Ctrl-click (Windows/Linux) does not open the link in a new browser tab. Instead, the click is intercepted by the sub-pages router and the navigation happens in the same tab. This breaks the standard browser behavior users expect from modifier-key clicks on links.

def root():
    ui.sub_pages({
        '/': lambda: ui.link('Go to other page', '/other'),
        '/other': lambda: ui.label('This is the other page'),
    })

Implementation

The global click event listener in sub_pages.js calls e.preventDefault() for all clicks on internal links (those starting with /) to handle client-side routing. However, it did not check for modifier keys before intercepting.

The fix adds an early return when any modifier key (metaKey, ctrlKey, shiftKey, altKey) is held during the click, allowing the browser to handle these events natively (e.g., open in new tab, open in new window, download).

Progress

• The PR title is a short phrase starting with a verb like "Add ...", "Fix ...", "Update ...", "Remove ...", etc.
• The implementation is complete.
• This PR does not address a security issue.
• Pytests have been added.
• Documentation is not necessary.
• No breaking changes to the public API.

[evnchn]

Nice. I was wondering where's the coverage for the "Windows key" (Windows) and "Super key" (for Linux). Turns out both in metaKey, so yeah we got all keys.

Claude Code is saying that typically SPA do not exclude alt-click, but I think we should, since we have nothing to do with alt-click and we should not "occupy" it as such.

It'd be a different story if we want to add alt-click function, which I don't see in the long-term future.