Visual Studio 2015 keeps switching startup project

Resharper , run configuration was the culprit in this case. The change was caused by that

ReSharper -> Tools -> Run Configurations...

I like to delete all the run configurations I find in that menu.

I had this experience after using the Resharper command 'Create Run Configuration' on a unit-test project. Thereafter, using F5 would always try (unsuccessfully) to run that project, and switch my default. I worked around it by running 'Create Run Configuration' again on the project I want to run.

I reported this as a bug at https://resharper-support.jetbrains.com/hc/en-us/requests/853379?flash_digest=cf3e117b89155bb0dcffa49c21f732b49d461f49