Clicking on a maximized application with the mouse at the top deselects it

Question

I've been having an odd issue, while minor is rather annoying using Windows 7 x64. For some reason starting a few weeks ago, if my mouse pointer is at the very top of the screen and I click on an window that is maximized, it does not select it and actually deselects it if it is the active window; I need to move the mouse pointer further down on the title bar to select the application.

It seems as though while an application is maximized it doesn't actually include the top row of pixels. This has not always been the case as I have always moved the mouse to the very top of the screen when selecting another window to be active. Any ideas what would be causing this?

Clicking on the top bar selects window per usual:

Clicking on the top bar with mouse pointer at top of screen deselects the window (if active) or does not select it (if not active):

Notice the color of the window's bar, in the second screenshot it is lighter (meaning not active).

Answer 1

Interestingly enough, this seems to no longer be an issue. The Anti-virus program was automatically run and although it didn't find anything, after rebooting I can again have the mouse all the way at the top of the screen and still select the top window.