Attachment coming up. The gif in question has a background that looks like it should be transparent (in GIMP), but flickers between transparent and black as the image animates... Originally reported on Windows, and I'm seeing it on Linux with current trunk builds.
This gif is constructed with only one graphics control extension which specifies the timing and the transparent index. It appears whatever app generated the image expected the control extension to apply to the three subsequent images. The gif specification states that the scope of the extension is the graphics rendering block that follows it. Per spec we clear the extension state at the end of sub block, so only one frame will show with the transparent background. Leaving open for paper to give a second opinion, but it looks like our behavior is correct.
tor is correct, resolving to INVALID Transparency (and any other control block properties) only apply to the frame it's specified on and does not carry over to future frames. Only the first frame contains a control block, the remaining frames do not, so they don't have a transparency.