Plank steals more focus then it should when compositing manager is on.

I'm using Plank version 0.5.0, on Gentoo, with Fluxbox window-manager.
For compositing I use compton.
The issue is that when compton is turned on plank seems to steel focus in a bigger area than it physically occupies. It gets even worse when hiding is on, as this zone doesn't seems to get smaller. This invisible zone overlaps my open windows and doesn't allow me to press any buttons in windows in the vicinity of plank.
I have confirmed that this is happening using xev: I place it close to plank and when moving my mouse, somewhere in the middle of the xev mouse event window it reports a lose focus.
The second way I have confirmed this is using xprop:
when compton is off it reports the following:

WM_NORMAL_HINTS(WM_SIZE_HINTS):
                program specified location: 0, 0
                program specified minimum size: 57 by 732
                program specified maximum size: 57 by 732

when I turn on compton i get:

WM_NORMAL_HINTS(WM_SIZE_HINTS):
                program specified location: 0, 0
                program specified minimum size: 128 by 768
                program specified maximum size: 128 by 768

I have also checked and the same thing happens using xcompmgr for compositing.