Follow the song in the lyrics

Synced LRC highlights the playing line, keeps it centred and seeks on click.
Plain text scrolls continuously with a nudge knob instead — a highlight on
guessed timings turns guaranteed drift into what looks like a bug. Seuraa
kappaletta turns following off without losing the highlight, and scrolling by
hand turns it off too.

Fixes the scroll landing in the wrong place (offsetTop measured from a
different coordinate space than the box it was applied to) and the fader
shifting the deck sideways at score 100 (auto-sized grid columns plus a
readout spanning both).

lyrics.go

@@ -9,6 +9,8 @@ import (
 	"net/http"
 	"net/url"
 	"regexp"
+	"sort"
+	"strconv"
 	"strings"
 	"time"
 )
@@ -49,6 +51,51 @@ var lyricsClient = &http.Client{Timeout: lyricsTimout}
 // nobody wants to read [00:11.74] at the start of every line.
 var lrcStamp = regexp.MustCompile(`^(\[\d{1,2}:\d{2}(?:[.:]\d{1,3})?\]\s*)+`)
 
+// A line of synced lyrics: the seconds it starts at, and the words.
+type lyricLine struct {
+	At   float64
+	Text string
+}
+
+var lrcOne = regexp.MustCompile(`\[(\d{1,2}):(\d{2})(?:[.:](\d{1,3}))?\]`)
+
+// parseLRC returns nil for plain text, which is the signal to scroll continuously instead of
+// highlighting: a line-by-line highlight on guessed timings makes every second of drift read as a
+// bug.
+func parseLRC(s string) []lyricLine {
+	if !strings.HasPrefix(strings.TrimSpace(s), "[") {
+		return nil
+	}
+	var out []lyricLine
+	for _, raw := range strings.Split(s, "\n") {
+		stamps := lrcOne.FindAllStringSubmatch(raw, -1)
+		if len(stamps) == 0 {
+			continue
+		}
+		text := strings.TrimSpace(lrcStamp.ReplaceAllString(raw, ""))
+		// One line can carry several timestamps when a refrain repeats.
+		for _, m := range stamps {
+			min, _ := strconv.Atoi(m[1])
+			sec, _ := strconv.Atoi(m[2])
+			at := float64(min*60 + sec)
+			if m[3] != "" {
+				frac, _ := strconv.Atoi(m[3])
+				switch len(m[3]) {
+				case 1:
+					at += float64(frac) / 10
+				case 2:
+					at += float64(frac) / 100
+				default:
+					at += float64(frac) / 1000
+				}
+			}
+			out = append(out, lyricLine{At: at, Text: text})
+		}
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].At < out[j].At })
+	return out
+}
+
 func stripLRC(s string) string {
 	if !strings.HasPrefix(strings.TrimSpace(s), "[") {
 		return s