I checked the following with builds 3290 and 3296:
Now in build 3290, the active editor is A.java whereas in build 3296, it is C.java.
Apparently the former policy "When closing one editor, make the one to the left active" has been replaced by something like "When closing one editor, make the most recently opened one active" (just a guess).
I think this change is very unfortunate because for the user, it is close to impossible to predict which editor will be active when closing another one. Especially when you have several rows of editor tabs (as I usually do), you lose a lot of time trying to figure out what is being displayed.
Please, Jetbrains, please restore the old behavior (or at least make it configurable).