Flickering in Cycles Render view mode /"Path Tracing Sample"

When in Cycles render view mode my viewport image continuously flickers, and I see a “Path Tracing Sample” notice in the upper left. This happens both in CPU and GPU (it just happens FASTER in gpu).

Side note: when the viewport is maximized (ctrl+space) the view is never re-sized to the bigger area available, and the flickering continues.

How does one avoid this effect when using Cycles?