Refine tooltip locations to minimize covering other controls

[jvine]

Tooltips are currently separated from their associated icon buttons by 14px, which is precisely enough to cover the control below the current one, or for the association to be disrupted by intervening borders and separators.

For desktop mode, please reduce the separation to 0px. That is, the tooltip will touch the control it is associated with, when the viewport is >600px.

For <600px, it's not clear how this adjustment will affect mobile use and touch trigger of tooltips, so I'm not proposing to change that setting for now.