I think this is a bug.
The panel variant that I use for mobile sites is still being used even with ?nomobile=true (although the correct desktop theme *is* being used). I can add another selection rule to get around this:
So, to duplicate:
1. Create a mobile theme
2. Set up mobile tools to redirect to domain.com/mobile and to use mobile theme
3. Create Home Page panels variant with selection rule "Mobile Tools" and select all devices
- make sure variant content is actually different from desktop version
4. In mobile device, visit domain.com/mobile?nomobile=true
This results, for me, in the desktop theme being used but still using the mobile Home Page variant. Adding the above referenced additional selection rule bypasses this.
If this truly is a bug, I'd be glad to look into a fix. If not, can someone tell me what I'm doing wrong?