From fcda9ebd5e3b3e2dbaba376a763cdab0f560d357 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Thu, 15 Oct 2026 09:46:18 +0000
Subject: [PATCH] fix(lyrics): keep explicit [bg:] trailing ends as line ends

A background layer's trailing marker is exact source timing. Without it on
Line.End, normalization treats a terminal bg cue end equal to the next line
start as a next-line fallback and drops it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016L1sBqsygMkdhf583gTEnE
---
 model/lyrics_lrc.go      | 19 +++++++++++++++----
 model/lyrics_lrc_test.go |  8 ++++++++
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/model/lyrics_lrc.go b/model/lyrics_lrc.go
index eaa35d9ee..09dd62627 100644
--- a/model/lyrics_lrc.go
+++ b/model/lyrics_lrc.go
@@ -53,8 +53,12 @@ func parseLRC(language, text string) (*Lyrics, error) {
 	flushLine := func() {
 		value, cues, end := parseEnhancedLine(priorLine)
 		var merged bool
-		value, cues, merged = mergeBgLayers(value, cues, pendingBg)
+		var bgEnd *int64
+		value, cues, bgEnd, merged = mergeBgLayers(value, cues, pendingBg)
 		hasBg = hasBg || merged
+		if bgEnd != nil && (end == nil || *bgEnd > *end) {
+			end = bgEnd
+		}
 		for idx := range timestamps {
 			startCopy := timestamps[idx]
 			shift := timestamps[idx] - timestamps[0]
@@ -207,13 +211,20 @@ func parseLRC(language, text string) (*Lyrics, error) {
 
 // mergeBgLayers appends parsed [bg:] content to a line's value and cues as a
 // background agent layer, mirroring the TTML background-vocal representation.
-func mergeBgLayers(value string, cues []Cue, bgLines []string) (string, []Cue, bool) {
+// It also returns the latest explicit trailing end among the merged layers, so
+// an exact source end is not mistaken for a next-line fallback during
+// normalization.
+func mergeBgLayers(value string, cues []Cue, bgLines []string) (string, []Cue, *int64, bool) {
 	merged := false
+	var latestEnd *int64
 	for _, bg := range bgLines {
-		bgValue, bgCues, _ := parseEnhancedLine(bg)
+		bgValue, bgCues, bgEnd := parseEnhancedLine(bg)
 		if len(bgCues) == 0 {
 			continue
 		}
+		if bgEnd != nil && (latestEnd == nil || *bgEnd > *latestEnd) {
+			latestEnd = bgEnd
+		}
 		offset := 0
 		if value != "" {
 			offset = len(value) + 1
@@ -228,7 +239,7 @@ func mergeBgLayers(value string, cues []Cue, bgLines []string) (string, []Cue, b
 		}
 		merged = true
 	}
-	return value, cues, merged
+	return value, cues, latestEnd, merged
 }
 
 func fillMainAgentID(lines []Line) {
diff --git a/model/lyrics_lrc_test.go b/model/lyrics_lrc_test.go
index 700e49a6e..ea52d1a4d 100644
--- a/model/lyrics_lrc_test.go
+++ b/model/lyrics_lrc_test.go
@@ -183,6 +183,14 @@ var _ = Describe("parseLRC", func() {
 		Expect(lyrics.Line[1].Value).To(Equal("Next"))
 	})
 
+	It("keeps an explicit [bg:] trailing end that equals the next line start", func() {
+		lyrics, err := parseLRC("xxx", "[00:01.00]Main\n[bg: <00:01.50>Ooh<00:03.00>]\n[00:03.00]Next")
+		Expect(err).ToNot(HaveOccurred())
+		Expect(lyrics.Line).To(HaveLen(2))
+		Expect(lyrics.Line[0].Value).To(Equal("Main Ooh"))
+		Expect(lyrics.Line[0].End).To(Equal(new(int64(3000))))
+	})
+
 	It("should skip a [bg:] line that has no inline timing", func() {
 		lyrics, err := parseLRC("xxx", "[00:01.00]Hi\n[bg: untimed]\n[00:03.00]Next")
 		Expect(err).ToNot(HaveOccurred())