When you press command + F12 in a class, you normally see all members in an unfolded tree. Now, this consistently opens folded. I updated to the latest major today and later on I noticed this issue. I already tried 'invalidate caches / restart'.
How can I make PhpStorm open this pop up with the tree unfolded like it used to?
You cannot -- it's a regression.
https://youtrack.jetbrains.com/issue/WI-38965 -- will be fixed for next version (2017.3.1).
P.S. No need to ask here and file a bug report to PhpStorm's Issue Tracker at the same time -- If you are so confident that it's a bug .. then why asking here?