Add an Orthogonal mode to the JS API and AGOL measure widget. And add snapping; with programmer and/or user-specified layers to snap to. Make both these features accessible by key combo and/or also by graphical user interface (might be more discoverable; but may be nicer to tuck away via key combo for pro apps for power users) I also suggest adding these both to the Editing widget. Would like this in 3.x (because we use WAB 2D) but I would imagine it would probably only make it to the 4.x.
Ortho mode in measurement was actually one thing I liked in AutoCad that seems to just work easily and nicely.
Jianxia Song Here is my user story example - I wanted to measure a 30-foot easement off a property line at a few spots. I would like to measure a straight line at 90 degrees at a few spots, and of course starting at the property line (snapped to). Currently is "eyeballed", which is not optimal.