Seems to be an insufficient repaint area - pot. related to bug #312168.

Wild guess from your patch: don't remove the shadows before, but *after* the hiding (eventually "later", ie. in the deconstructor or on the destroyed signal) to avoid confusations between the deleted window size and the "dirty" area.

- Thomas

On December 25th, 2012, 12:14 a.m., Xuetian Weng wrote:

Well. I'm not an expert in kwin related stuff, this patch is basically revert 157a06e46f46ba83ba37a93b400647f4886e18c9, but I don't know the real reason for that.

And actually, remove by handle is not necessary, since dialogshadows already connect to the widget's destroyed signal, hence it will handle that gracefully.

seems it fixed, but not sure it's the right way.