I'm writing again because of the bug
http://www.jetbrains.net/jira/browse/IDEA-553 (A file should be synchronized with disk evertime it gets activated - Not only checked on saving).
This bug exists since a 3.x-Version of intellij. For me its very annoying because I have to diff the files manually because of this bug (like right now).
And I should not be the only one with this problem:
(This should be also fixed with a synchronize-file-on-file-activation-feature)