Currently in Firefox after you've picked an element we show dotted lines to describe its relationship in respect to the viewport. They're considered somewhat obtrusive, as described by the first comment in Bug 1128209. Sketch uses the <alt> key to describe elements' relationships to each other once you've put a lock on element, as described in the attachment. People loved this about Sketch when it first came out and the Adobe products have all rushed to follow in Sketch's footsteps for this feature. My recommendation as part one in cleaning up the way our locks on elements look is to include the <alt> functionality so that we can remove the dotted lines that we currently show be default on locked elements.

This is an enhancement we're likely to work on in the scope of the "Visual Tools" project in Q3 this year.