For each edge an additional "approach" area window is created. When the
mouse enters this approach window, it gets unmapped and a mouse polling
interval is started. If the mouse leaves the approach area again, the
window gets mapped again and the mouse polling is stopped.
During the approaching a signal is emitted with a factor in [0.0,1.0] to
describe how close the mouse is to the edge. 0.0 means far away, 1.0
means triggering the edge. This signal is passed to the effects to allow
using this information. E.g. to provide a glow corner effect or to make
use of it in the cube animation effect to start the animation on desktop
switch.
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