The shadow effect is known to be broken since at least 4.5.
It is unfortunately in a state which makes it difficult to maintain
and the architecture has some serious drawbacks. Therefore it is the
best solution to replace the effect with a new and better
implementation. For more information about the new implementation
please see the discussion on KWin mailinglist:
http://lists.kde.org/?l=kwin&m=129607406517609&w=2

This also "fixes" all existing bug reports about the shadow effect.
Most of the bugs will really be fixed when the new shadow system is
implemented, if not it is a new bug and a new report should be created
for it.

Please excuse that we go this unnormal approach to mark bugs as
fixed with code removal.
BUG: 164084
BUG: 160948
BUG: 189241
BUG: 229164
BUG: 258663
BUG: 216709
BUG: 243890
FIXED-IN: 4.7.0