Let's say that there is a solution that contains two projects (Project1 and Project2).
Project1 is set as a StartUp Project (its name is displayed in a bold font). I double-click some file in Project2 to open it. The file opens, but something else happens too - Project2 gets set as a StartUp Project.
I tried to find an option in configuration to change it, but I found none.
Can this feature (though it's more like a bug to me) be disabled?
The way to select a startup project is described in Sara Ford's blog "Visual Studio Tip of the Day" (highly recommended). She has a post there about setting up StartUp projects. Essentially there are 2 ways, the easiest one being right-clicking on the desired project, and choosing "Set As StartUp Project". That prevents other projects from becoming the StartUp project, even if you click on one their files.
Check your Visual Studio options for the following check box:
Projects and Solutions - Build and Run - For new solutions use the currently selected project as the startup project.
Uncheck that and see if the behavior changes.