This seems to be a Win 10 Pro issue as well as the Insider builds, I’m having the same issue and all the past UE4 bug reports triangulate to a select number of windows build - from what I can find, Insider and Pro. Pro being the recent build to have this issue.
Be good to get some info on this?
Update: Possible short term fix and workaround solution to the above issue.
it seems in my case to be related to an extended-second screen running at a different, in my case, lower resolution to that of screen 01.
I placed the UE4 editor into the second screen and the viewport/game window in the screen 01.- Only working on one screen
I matched (over clocked via graphic settings) screen 02 to match screen 01 “Can`t right-click” Issue fixed
This actually makes sense when you think about the editor being split into two entirely different resolutions
I still need to run through a few more repeated scenarios before confirming anything but will be worth a try your end (The user).
Let us know
Steve