Please describe the problem:
When using the playback feature, the bottom row of pixels doesn't behave
properly with optimized animated GIFs.

Steps to reproduce:
1.  Play an optimized animated GIF where the bottom row of pixels has changes.
2.  Watch the very bottom row of pixels on the playback.

Actual results:
There seems to be an unexpected delay in the motion of it, of which doesn't
update until after about 20 frames.

Expected results:
The playback should be just like it would appear in a browser, the bottom row of
pixels should change only when there is a change.

Does this happen every time?
Depends.  It only occurs when an optimized animated GIF is played.  When you
unoptimize it, it doesn't happen any more.

Other information:
I have a screenshot and a sample.  The screenshot shows the effect and how it
compares to one of the frames.  The sample is the same file shown in the
screenshot (save it to disk then open it in GIMP).

http://www.ulillillia.us/temporary/animationplaybackbug.png - the screenshot
http://www.ulillillia.us/images/holy1.gif - the sample, of which appears
normally in the browser.