The phenomenon, which you described as "a large band of shadow", is likely because the window manager Fluxbox is not using any compositing manager.
Modern docks such as
plank relies on a compositing manager for its appearance rendering and animation effects. This is a default, therefore compositing is required for these docks and there is no way to skip this requirement (when using earlier releases of
Latest solution: Update
As release 0.10.1, the dock seems to support both compositing and non-compositing environments properly. I have tested this release on Xfce 4.10 with Xfwm.
Solution 1: Compositing
You will need a compositing manager that works with Fluxbox. According to this Wiki, the only option with Fluxbox is
compton. Not much details, but someone had managed.
Solution 2: Non-compositing
Use other dock that works even without compositing, such as
cairo-dock. The latter seems to work better with compositing. These docks will have solid background when used in non-compositing mode. No compositing means no transparency.
I have experienced this phenomenon when I had tested
plank during its early development in GNOME 2 and Xfce environments. These X environments had their built-in compositing managers, Metacity and Xfwm, which can be turned on and off easily.
This area cannot be clicked through and any window under it, just like a whole part of the desktop, is thus unaccessible.
However, I didn't remember the shadow had prevented me from clicking the launchers on the dock (it was some time ago, so I can't recollect my testing experience entirely).
Fluxbox Wiki had this howto that explains how to use compositing with Fluxbox using
xcompmgr, which was said to be "nothing but eyecandy". Since its development had discontinued, its fork
compton took over the role, which notably known for its solution to "tear-free experience" in Xfce.
Dock with compositing and non-compositing
To describe the difference between two solutions above, I have used
avant-window-navigator in Xfce environment to demonstrate the effect.
In the first screenshot, the information dialog is partially visible under semi-transparent dock background and has drop down shadows around the information dialog itself.
In the second screenshot, the information dialog is no longer visible and hidden under the light grey solid background and no more shadows.