screen update is suspended during some operations

As reported on https://bugs.debian.org/934692 :

“I'm using compiz by default on my desktop. During some operations with thunderbird and synaptic, screen update is suspended (seemingly hangs) for a few seconds:

• when I start thunderbird, until new mail is retrieved.
• in synaptic, after selecting packages for install or upgrade, when I click on "apply" - until the installation is completed.

During those few seconds the mouse and keyboard are inoperative.”