From 129d51a1e6e24126f4b6296565faef4cbf8a267b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=BAlio=20C=C3=A9sar=20Batista?= <julio@terrastruct.com>
Date: Fri, 28 Apr 2023 18:26:25 -0300
Subject: [PATCH] fix gif pallete

---
 lib/xgif/xgif.go | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/lib/xgif/xgif.go b/lib/xgif/xgif.go
index 967b446fc..a0d21695a 100644
--- a/lib/xgif/xgif.go
+++ b/lib/xgif/xgif.go
@@ -25,7 +25,6 @@ import (
 )
 
 const INFINITE_LOOP = 0
-const BG_INDEX uint8 = 255
 
 var BG_COLOR = color.White
 
@@ -85,12 +84,19 @@ func AnimatePNGs(ms *xmain.State, pngs [][]byte, animIntervalMs int) ([]byte, er
 		left := (width - bounds.Dx()) / 2
 		right := left + bounds.Dx()
 
-		palettedImg.Palette[BG_INDEX] = BG_COLOR
+		var bgIndex int
+		if len(palettedImg.Palette) == 256 {
+			bgIndex = 255
+			palettedImg.Palette[255] = BG_COLOR
+		} else {
+			bgIndex = len(palettedImg.Palette)
+			palettedImg.Palette = append(palettedImg.Palette, BG_COLOR)
+		}
 		frame := image.NewPaletted(image.Rect(0, 0, width, height), palettedImg.Palette)
 		for x := 0; x < width; x++ {
 			for y := 0; y < height; y++ {
 				if x <= left || y <= top || x >= right || y >= bottom {
-					frame.SetColorIndex(x, y, BG_INDEX)
+					frame.SetColorIndex(x, y, uint8(bgIndex))
 				} else {
 					frame.SetColorIndex(x, y, palettedImg.ColorIndexAt(x-left, y-top))
 				}
@@ -115,8 +121,10 @@ func Validate(gifBytes []byte, nFrames int, intervalMS int) error {
 		return err
 	}
 
-	if anim.LoopCount != INFINITE_LOOP {
+	if nFrames > 1 && anim.LoopCount != INFINITE_LOOP {
 		return fmt.Errorf("expected infinite loop, got=%d", anim.LoopCount)
+	} else if nFrames == 1 && anim.LoopCount != -1 {
+		return fmt.Errorf("wrong loop count for single frame gif, got=%d", anim.LoopCount)
 	}
 
 	if len(anim.Image) != nFrames {