This is a tracking issue against saitoha#61.

This issue has a bit of a storied history already.

• @schauveau opens it in 2017;
• @alxpettit reproduces in 2019;
• @auppal opens Fix decoding of high-color sixel images when the palette changes. saitoha/libsixel#147 in February;
• I merge Fix decoding of high-color sixel images when the palette changes. #1 with the note in Notification of fork at libsixel/libsixel (libsixel/libsixelのフォークのお知らせ). This project is unmaintained. (管理者 @saitoha が不在です。) saitoha/libsixel#154 "I found the loss of bgindex in certain functions somewhat troubling, but until there's an API for that I don't think it even matters."
• I revert @auppal (49f4326) because context.bgimage is actually useful and can't be removed.

That's where it stands…maybe @auppal will come back and fix it, if not I probably will end up doing so.