To simplify things and help our users to be more productive, we have archived the current forum and focus our efforts on helping developers on Stack Overflow. You can post new questions on Stack Overflow or join our Discord channel.
Showing a notification when a Button is clicked seems to work perfectly for me with Vaadin 7 alpha 2 running on Jetty 6.1 in Firefox 11 (with OS X 10.6 and the bundled 1.6 JDK).
Some things you could do to possible help pinpoint the problem is to test with different browsers or application servers (Jetty and Tomcat usually the simplest to get up and running for testing). You could also see what happens in you enable the disableOnClick feature of the Button.