I am having this issue and I am totally confused by what is going on.
Our current client machines are windows 7, and everything works fine with them, however we are thinking of moving to windows 8.1, so I got a new Dell 2 in 1 laptop. When this problem with webmail hit.
Basic problem is my right click context menu on the folders list or the email messages list does not work on IE11.
I get this error in the F12 Developer tools :-
File: masks.ashx, Line: 283, Column: 42
Unable to get property 'top' of undefined or null reference
The windows 7 machines are using the same version of IE11 and the right click works fine.
Smartermail Server is 2008r2 running the latest v13.3, I also created a new test server running 2012r2 VM, and installed the latest smartermail on that - and the webmail for that server has the same problem!!!
I am not sure if this problem is related to the fact the Dell machine has a touchscreen or multi touch pad.
Help!! Thanks for reading.