Logout User with multiple tabs open in ASP.Net

if the user has multple tabs open, the other tabs don't logout until there is postback on the screen.  I want when a user clicks on another tab it immediately logs out.

Currently if a user logouts on the current tab and goes to another tab, they can see the page until they postback the page and then it logs them out, how do i modify the above code where if the user clicks on another tab (while logged-out) it logs the user out so they can't see the page at all and logs them out.