the user operation is waiting for background work to complete

In my case, I increased values of -Xms and -Xmx args to 512 and 2048 respectively in eclipse.ini file. You can increase it according to you or according to your system ram.
And then restart eclipse.

Eclipse can have a tendency to hang sometimes for no apparent reason (usually because of memory). Just try restarting it and see if the problem persists. I've encountered this before and restarting Eclipse or the PC altogether usually solves it.

Go to Project Menu Uncheck the Build automatically and Restart the Eclipse/STS IDE.