Explained previously ( http://www.byond.com/forum/post/2692653 ) and test case was sent.

For games that use HUD overflow, filters take HUD into account to determine the center for radial blur, layering and other effects.
It's rarely a desirable behavior: most effects are meant for the game, not it's HUD.

I also suspect that there's a hit on performance if the area that filters apply to is unnecessarily extended by the overflow.

Manually re-adjusting filters is a needless fuss and gets much harder when rescaling is involved, or the overflow changes during the game.

Could be solved by a flag.