Javascript Event Handling And Page Load

I understand that when loading a page, the html parser is suspended until any script that it comes across is executed. i also believe that certain browsers will allow user input for those parts rendered whilst the rest of the page is still being parsed. suppose that a user clicks a button that is visible and 'active' and fires off some lengthy javascript execution causing the ui to 'freeze' for a while. my question is, on a page that is still being parsed but allows user input on those parts parsed and rendered, will the parsing of the rest of the page cease whilst the event handler (onclick) is working or will the user see the page render simultaneously as the event handler does its job?