I fixed this problem by turning on the automatic synchronization first and then turning it off. (This is in Tools|Options|Projects and Solutions|General Finished, “Track active projects in Solution Explorer”)
I use Microsoft Visual Studio Ultimate 2013, and it looks like this button is in a project (WPF application) Work in the same solution instead of working in other projects (server applications) in the same solution. Which VS setting may I have broken?
“Sync with Active Document” stopped working in VS 2015, including the button at the top of the Solution Explorer, and the shortcut (Ctl [,S).
I fixed this problem by turning on automatic synchronization first, then turning off and then turning. (This is done in Tools|Options|Projects and Solutions|General, “Track Activity in Solution Explorer Item”)