Simple and super to let on the top chosen windows.
To keep the look on the content of a window
The principle is very simple. The program icon goes in the tray bar. A click upon it, and the cursor looks like the nail drawn on the icon. With this nail, you click on a window which stay on top, until it receives a second nail click, or it is closed.
Several windows may so be placed, the last nailed being on top of all.