I've also seen this. Not sure if it has something to do with updating java, but I lost the ability to undo typing. ctrl z doesn't work.
I tried changing preferences, undo/redo are working fine for me.
There is a bug https://bugs.adobe.com/jira/browse/FB-29897 which was closed because it was not reproducible consistently. Please add your comments/'steps to reproduce' to the bug.
Thanks for the adding comments in the issue https://bugs.adobe.com/jira/browse/FB-29897. The problem is faced whenever preferences are changed and there is an active editor opened.
I have re-ropened the issue.
if you close your project, and then re-open it you'll get your functionality back ... though by then it may be too late.