Single click between apps does not change focus

This problem is kind of difficult to describe and I don't know the correct lingo, so I had trouble searching for another thread that may be asking the same question.

Situation: I have two applications open (it happens with any combination of apps), let's say TextEdit and Calculator. The 'focus' is on Calculator. So I can type in numbers from the keyboard, or click the mouse on the Calculator numbers and it computes. No problem. But, if I want to then bring 'focus' to the TextEdit window -- to the extent so I can type something -- when I move the mouse anywhere over the TextEdit window and single click, it does not bring the focus to TextEdit and it stays greyed-out; the focus remains on Calculator.

In order to get it to bring focus to TextEdit so I can type, I have to click twice on the app. However, I cannot double-click on it, or it will minimize the window. I have to click once, wait a few seconds so it won't register as a doubclick, and then click again. This is incredibly frustrating, slows down my work flow, and I can't for the life of me figure out how to change it. It has been happening for quite some time, and I think changed after I downloaded some sort of new update, which one I can not remember unfortunately.

Again, this happens with ANY two applications. I am aware of being able to use command+tab to switch between apps and I use that frequently, but sometimes using my mouse to arrange windows or other actions is necessary.

Any help would be very much appreciated!

Thanks!