Bug 984407 - Reflows on focus/blur in e.me

Same patch as #17295, no clue what happened.

Here is the comment from the other issue:

> Perhaps we can remove updateY() altogether and implement a getter instead. Something like:

No, the thing is that we should only ever call `scrollTop` the moment that a `scroll` event comes in on the element. Because at that moment retrieving the info is 'free'. Whereas normally it causes a sync reflow. 
So we really need to cache here.

> Side effect - when returning to homescreen, the Evme searchbar overlaps the grid icons due to it's 8rem height, not allowing to click on them.

Yeah, that's not good ;-) I've fixed it in the last commit. The div is still too high but it doesn't steal pointer events anymore.