Browser Windows

I have been having a rather odd problem lately when using both IE and Chrome. After a short period of time using a browser window, it will suddenly 'deselect' itself, ie I can be typing or highlighting something within a browser window using the mouse/keyboard and suddenly my typing and highlighting aren't working because the window isn't selected. I have to reselect the window to carry on what I'm doing. Particularly annoying when I'm typing. It's happened three times when I'm making this post!

I think it's changing the focus from the window to the system tray because *sometimes* as I'm typing, the network icon shows I have been disconnected from the internet.

I'm using 64 bit Windows 7 Professional on a Lenovo machine.

I've had a look for anyone with a similar problem, and I've noticed people get disappearing cursors but they don't mention their problem affecting a window even when they're not typing.

Can anyone suggest a possible cause?

What type of mouse and keyboard/ Wireless/ built in as in a laptop ?
Fill in your system spec's page with the link above my signature picture will help others,
Loosing focus can be caused by almost anything updating/ Security..........
https://www.sevenforums.com/tutorials/220165-troubleshooting-steps-windows-7-a.html
https://www.sevenforums.com/tutorials/11728-optimize-windows-7-a.html

My first thought would be the mouse,
Do you have a wired one to test with ?