I think you can go either way in terms of 'correctness', but there is a subtle change in meaning.
- "Now you don't watch your screen as often as I'm glancing back at mine." - this is an expression denoting what the 'you' does on a regular basis.
- "Now you're not watching your screen as often as I'm glancing back at mine." - this expression seems to denote (and just barely, as they are rather close) that what 'you' does is kind of what 'I' do at the same time.
I would use the two sentences interchangeably, but there is a really, really slight difference there, at least how I'm reading the sentences.