After upgrading to IDEA 2017.3.4 I have found a new file in the .idea folder: hydra.xml. Its contents:
<?xml version="1.0" encoding="UTF-8"?>
<option name="projectRoot" value="C:\Some\absolute\path\to\my\project" />
I have spend some time to find out who creates this file, so it would be nice to have some clues that it's some of Scala plug-in configuration.
Should it be added to VCS or ignored?
Note that it contains absolute path to my project (should I report an issue?).
UPD: I have created the issue after a while: https://youtrack.jetbrains.com/issue/SCL-13438