Spun off from bug 376446 - the animated GIF in the URL shows a white (transparent, really) square during part of the animation which is not shown with other programs like IE7 and IrfanView. Bug 376446 comment 40 suggests this might be fixed by the patch for bug 317748, so marking dependent for now.

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9a5pre) Gecko/20070531 Minefield/3.0a5pre RyanVM asked me to see if the patch in bug 317748 fixed this for me. I applied the patch and can confirm it fixes this issue.

The following code in imgContainer.cpp SetMaskVisibility is suspicious: 928 asqueella 1.45 PRUint8* alphaData; 929 PRUint32 alphaDataLength; 930 const PRUint8 setMaskTo = aVisible ? 0xFF : 0x00; 931 932 aFrame->LockImageData(); 933 nsresult res = aFrame->GetImageData(&alphaData, &alphaDataLength); 934 if (NS_SUCCEEDED(res)) { 935 #ifdef IS_LITTLE_ENDIAN 936 alphaData += aY*frameWidth*4 + 3; 937 #else 938 alphaData += aY*frameWidth*4; 939 #endif 940 for (PRInt32 j = height; j > 0; --j) { 941 for (PRInt32 i = (aX+width-1)*4; i >= aX; i -= 4) { 942 alphaData[i] = setMaskTo; 943 } 944 alphaData += frameWidth*4; 945 } 946 } 947 aFrame->UnlockImageData(); Especially the line: 941 for (PRInt32 i = (aX+width-1)*4; i >= aX; i -= 4) { i is column position multiplied by 4 (so that it steps by the 4 bytes per pixel). But the end check: i > aX is then not right, and should be i >= aX*4. Instead of trying to fix this buggy code, bug 317748 completely removes this, so let we do that.

Fixed by the patch for bug 317748.