After waking up from suspend, I see these half-transparent window handles, but I don't know how to activate it.
Sometimes I can deactivate it by tapping the touchpad.
What is the function of this?
How to activate it?
That is the Unity Mt Grab Handles ( a compiz plugin). It gives you a move point (center) & resize handles.
To use - open the plugin in ccsm (compizconfig-settings-manager) & set a binding either for 'toogle handles' & or 'show handles'
It's value on a desktop/laptop install is questionable, there are easier ways to move & or resize windows & the plugin has been known to cause issues
If you have ccsm installed & wish to go directly to the plugin run
Edit: if deciding to use, - keep in mind that ccsm does not check for general keyboard binding conflicts so choose wisely
I believe that enables you to scroll the window by moving the mouse cursor. I don't know how to activate it, though.