I am using Lubuntu 18.04.
Sometimes, when I am using Firefox to browse webpages or some other applications, clicking ( left, right and middle) by my mouse or touchpad stops working. It is as if there were something else which is invisible, stands between the screen and the Firefox or other application's window, and steals clicks away. I can still move the cursor using my mouse or touchpad, and I can still use my keyboard to type any key and to switch to other application's window (via Alt + Tab).
In that situtation, if I log into another user's session without logging out the current one, the problem doesn't exist in the other user's session. But if I then switch back to the previous session of the original user, the problem remains.
What may cause the problem? It is unlikely the drivers or modules for the mouse or touchpad, since clicking after switching to another user's session still works. Can the cause come from the window manager openbox, lxsession, or something else?
What shall I do to solve the problem? Restarting Lubuntu is the least thing I want to try, every time when the problem occurs.