I mostly work on a retina display iMac (El Capitan 10.11.16, SketchUp 16.1.1451) and I have an issue where the Tape Measure tool does not find the edge I am clicking on but rather picks some other, and as far as I can tell, random edge. In the attached image “A” is where I wanted to pull a guide line from and “B” is where SketchUp selected.
Once this occurs the best way to get things back to normal is to hit the space bar and then repack the Tape Measure tool (I do this with the T shortcut). This happens quite often but only on my 5K iMac. I have a 13" MacBook Pro with a retina display and I don’t have the problem on that machine. SketchUp on that laptop is set up exactly the same with the same versions of OS X and SketchUp. doesn’t seem to matter how big the file is either. This did not happen in SketchUp 2015.
An associate of mine also has the same issue on his 5K iMac running SketchUp 2016.
SketchUp Pro 2016, 15 year SketchUp user starting with SketchUp 1.3.71, 6 year Mac user