In my case this was the solution: Go to Settings on the main menu and then to Appearance settings. Choose the Defaults tab and click one of the choices. Problem gone, but I don't know why or what happened in the first place.
My Pi 4 has just done this to me again and I am not accessing it through VNC. The problem is as annoying though and even 'on the desktop' using the defaults method is failiing.
For anyone who isn;t comfortable fiddling around with config files (for example - beginners), and who tried the 'defaults' method. I just discovered with the latest kernel (at Dec 2019) if I choose the Large Screens default, I get my title bar actions back - UNTIL - I edit the double click on title bar on Openbox config. Then I am hack to locked up and can't move, resize or click on title bar stuff. But if I click on Medium Screens default, all seems ok again.
I have no idea whether this is a coincidence since I upgraded my kernel, because until today I had not had this problem again for some weeks.
For now I am about to write a little script and whack it on the desktop or the panel so I can click on it and copy the file across with a click (or two if the settings have gone back to defaults
A script to sutomate what ReddTedd59 suggests, should easily solve my problem and might be something to consider for anyone accessing remotely through VNC.