client/web: open new window if iframed

Previously, we were only breaking out of iframes when accessing the
login client over a local IP address (where viewerIdentity is not set).
We need to also handle the case where the user is accessing the login
client over the Tailscale IP, and similarly break out of the iframe when
logging into the management client.

Updates #10261

Signed-off-by: Will Norris <will@tailscale.com>

client/web/src/components/login-toggle.tsx

@@ -126,7 +126,14 @@ function LoginPopoverContent({
 
   const handleSignInClick = useCallback(() => {
     if (auth.viewerIdentity) {
-      newSession()
+      if (window.self !== window.top) {
+        // if we're inside an iframe, start session in new window
+        let url = new URL(window.location.href)
+        url.searchParams.set("check", "now")
+        window.open(url, "_blank")
+      } else {
+        newSession()
+      }
     } else {
       // Must be connected over Tailscale to log in.
       // Send user to Tailscale IP and start check mode