Clicking Class in Editor Starts Another Visual Studio

I recently upgraded to UE4.18 and VS2017.

Previously when I clicked a C++ base class name in the top-right of a blueprint, it switched windows to my already-running Visual Studio, and opened a tab with that class. It was great.

Now, when build and run my project as normal from Visual Studio, and click on the base class name, UE4 tries to open up another instance of VS2017.

What can I do to fix this?

Happens to me in 2015, so you can rule out the VS version. No idea why it happens. I just avoid doing that. I search the code in VS.