This is an issue with C# and tabs instead of spaces when attempting to hit the metadata of an external assembly. It may be related the inferred position of the insertion point.
Others have documented this
There are a couple of Connect tickets here and here and a Github issue on this. There is also a discussion here.
Visual Studio 2015 Update 1
This issue is addressed in Update 1 so please install!
Poor workaround for RTM
The options dialog remembers the last page and remains there on subsequent opens. I have a keyboard shortcut to open the options pane quickly (Alt + o). I am temporarily changing to spaces, going to definition and then reverting to tabs before making any code changes. This workflow isn’t pretty but neither were the 3.0 Nuget issues in VS2015 either (Nuget 3 has improved to date).