Perhaps the reason for this behaviour is similar to what I have seen on two different Mac systems: Some events seem to double fire. DeepSOIC has worked on this, so you may try a current 0.19 version.
If the issue persists, try different Mouse Models. Please report back with your full FreeCAD infos included.
...And try with another Mouse Model as you also suggested above.
I did try another mouse mode last night. I just didn't want to reply back with that until after I tried the .19 pre-release. WIth a different mouse mode, I still have the same issue.
I may want to clarify something. The "switching to the tasks tab" seems to be only when I switch to the "Parts Design" workbench. I haven't tried all of the work benches but I will do that this weekend.