Irritating behaviour
I have a late-2012 Mac mini with Mojave (10.14.6). This is a very naive question, but it is about a behaviour that is intensely irritating and time-wasting.
If, while I am working, I accidentally allow the cursor to stray up into the area of the screen near the tool-bar, the screen goes light grey and the computer freezes. Until recently (and if my memory does not deceive me), as soon as I moved the cursor back into the working area, the screen became bright again and the computer came to life again. Now, the screen remains grey and the computer unresponsive until I click the mouse-button again.
In the scale of problems, this is not really serious; but it is irritating and time-wasting.
Am I right in thinking that this is a relatively new behaviour? Perhaps since an update? Is there any way of reverting to the old behaviour? I should be grateful for advice.