Possible race condition when clicking on a tab at the bottom of a page in view mode

This makes functional UI tests flickers.

Here's what is probably happening:

• the test opens a page in view mode
• the JS on that page (in docextra.vm) registers event observers for clicks on the tabs
• the test click on the history tab (for ex)
• the browser executes the JS that toggles the active tab for history first
• the browser then executes the toggle for the comment tab

Normally the toggle for the comments tab happens first but there could be a race condition that makes the one for the clicked tab happen first.

According to Sergiu we could fix by having a semaphore counter and discard events that have a counter id lower than the last received count.