I am using emacs 24.5.1 on Ubuntu 16.04. Sometimes parts of the buffer become invisible. For example, in the minibuffer the part that represents the command and partial argument can become invisible, such as c-x c-f failing to show "Find file: " and the directory string. In Perl I have found parts of the buffer that highlight different components such as variable names or programming language words might also become invisible, so the problem is not just about the minibuffer. The problem does not go away when using 'emacs -Q'. I used to use an older version of Ubuntu that is no longer supported, and the problem started appearing after I upgraded to 16.04.
Can you help me understand what is happening, and what I might do to debug it? I started looking at overlays and text properties, but I still don't understand them well enough. The characters can be seen when I backspace over them. I cannot predict when the problem will occur, and the current remedy is to exit emacs and start again.
Thanks,
Bob