Mouse cursor removes focus from windows

Hi, guys!

So a computer I have is presenting this weird bug (it's not the mouse)

When I turn the computer on, it works fine for some time, maybe minutes, maybe hours.

Suddenly, out of the blue, clicking with the mouse on most or all Windows takes the focus away as if I was clicking on another window or an empty space in the taskbar. You know, when a window is active and in focus, the title bar is darker and some X exit buttons look red, and when you click on another window, it loses focus and the title bar goes lighter/dimmer and the X loses the red color. This happens when clicking ON the current focused window.

• The windows are all responsive, I can still Alt+Tab to them and do things with the keyboard like writing and using keyboard shortcuts for the menus and buttons.
• Changing the mouse doesn't solve the issue.
• Not only that, accesing the computer remotely with another mouse (which works flawlessly on the other PC), produces the same effect.
• More over, using MouseKeys (Ctrl+Alt+NumBlk) to click with the NumPad also does the same effect.
• Virtual clicks made with AutoIt script do the same. So the mouse is fine, ruled out as I mentioned.

I have seen this years ago on some friend's PC and it had to do with some invisible/transparent Splash screen that failed to load. So you were really clicking the ghost window. But the area that failed to my friend was defined, in the center of the screen and the error happened with all windows and the desktop.

This bug tends to apply to all opened windows... but may happen to some, while others work fine at the same time, for instance, the Desktop may work, but Notepad may not. Or the Taskbar works but not the Start Menu. If you close any working window and open another instance or just open a new one, the new one wont work (mostly). So I, more or less, ruled out the posibility of a ghost screen... Unless it has to do with some weird stacking bug.

• I tried Killing all the disposable processes and Stopping all the non important services. That didn't fix the problem.
• The computer didn't get updates or new programs in the last many months.
• I run the Trend Antivirus and MalwareBytes without issues.

The only workaround I found that bypasses this bug for a while is pressing Ctrl+Alt+Del and canceling the "Change the password, etc" screen. That fixes the cursor until it happens again.