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).

static/lyrics.js

@@ -0,0 +1,135 @@
+// Lyrics that follow the audio. Two behaviours, because the two kinds of lyrics deserve different
+// treatment: real LRC timestamps get a line highlight, guessed timings get a continuous scroll and
+// a nudge knob. Progressive enhancement — without this file the lyrics are still readable text.
+(function () {
+  'use strict'
+
+  const quiet = window.matchMedia('(prefers-reduced-motion: reduce)').matches
+
+  // The audio element belonging to the same strip, falling back to the only one on the page.
+  const audioFor = (box) => {
+    const scope = box.closest('.strip') || box.closest('section') || document
+    return scope.querySelector('audio') || document.querySelector('audio')
+  }
+
+  // --- synced: highlight the line that is playing ---
+
+  function enhanceSynced(box) {
+    const audio = audioFor(box)
+    if (!audio) return
+    const lines = [...box.querySelectorAll('.lline')]
+    if (!lines.length) return
+    const times = lines.map((l) => Number(l.dataset.t))
+    let current = -1
+
+    // Following is what moves the box. Timestamps are somebody else's guess at where a line
+    // starts, so when they are off, the scrolling is the part that fights you — the highlight can
+    // stay. Remembered per song.
+    const follow = box.parentElement.querySelector('.follow input')
+    const key = 'lyricsfollow:' + box.dataset.song
+    if (follow && localStorage.getItem(key) === 'off') follow.checked = false
+    if (follow) {
+      follow.addEventListener('change', () => {
+        localStorage.setItem(key, follow.checked ? 'on' : 'off')
+      })
+    }
+
+    // Scrolling the box by hand turns following off: reading somewhere else is a clear statement
+    // that you do not want to be dragged back.
+    let selfScroll = false
+    box.addEventListener('scroll', () => {
+      if (selfScroll || !follow || !follow.checked) return
+      follow.checked = false
+      localStorage.setItem(key, 'off')
+    })
+
+    const show = (i) => {
+      if (i === current) return
+      if (lines[current]) lines[current].classList.remove('on')
+      current = i
+      const line = lines[i]
+      if (!line) return
+      line.classList.add('on')
+      if (follow && !follow.checked) return
+      // Measured against the box itself. offsetTop is relative to the nearest positioned ancestor,
+      // which is not this box, so using it scrolls to a position from a different coordinate space.
+      const boxRect = box.getBoundingClientRect()
+      const lineRect = line.getBoundingClientRect()
+      const target = box.scrollTop + (lineRect.top - boxRect.top)
+        - box.clientHeight / 2 + lineRect.height / 2
+      selfScroll = true
+      box.scrollTo({ top: target, behavior: quiet ? 'auto' : 'smooth' })
+      // Long enough for the smooth scroll to finish, so our own movement is not mistaken for the
+      // reader's.
+      setTimeout(() => { selfScroll = false }, 700)
+    }
+
+    audio.addEventListener('timeupdate', () => {
+      const t = audio.currentTime
+      let i = current
+      // Usually one step forward; a seek walks from wherever it lands.
+      if (i < 0 || times[i] > t) i = 0
+      while (i + 1 < times.length && times[i + 1] <= t) i++
+      if (times[i] <= t) show(i)
+    })
+
+    audio.addEventListener('seeked', () => {
+      current = -1
+    })
+
+    // Clicking a line seeks to it: the lyrics become a way to navigate the song.
+    lines.forEach((line, i) => {
+      line.addEventListener('click', () => {
+        audio.currentTime = times[i]
+        if (audio.paused) audio.play()
+      })
+    })
+  }
+
+  // --- plain: scroll the block in step with the audio ---
+
+  function enhancePlain(box) {
+    const audio = audioFor(box)
+    const inner = box.querySelector('.lscroll')
+    if (!audio || !inner) return
+
+    const nudge = box.parentElement.querySelector('.nudge input')
+    const readout = box.parentElement.querySelector('.nudge output')
+    const key = 'lyricsoffset:' + box.dataset.song
+    let offset = Number(localStorage.getItem(key) || 0)
+    if (nudge) {
+      nudge.value = offset
+      readout.value = offset + ' s'
+    }
+
+    const duration = () => Number(audio.duration) || Number(box.dataset.duration) || 0
+
+    // Position is a pure function of time, so a seek needs no bookkeeping and drift cannot
+    // accumulate the way it would with a timer.
+    const place = () => {
+      const total = duration()
+      const travel = inner.scrollHeight - box.clientHeight
+      if (total <= 0 || travel <= 0) return
+      const at = (audio.currentTime + offset) / total
+      box.scrollTop = Math.max(0, Math.min(travel, at * travel))
+    }
+
+    audio.addEventListener('timeupdate', place)
+    audio.addEventListener('seeked', place)
+    audio.addEventListener('loadedmetadata', place)
+
+    if (nudge) {
+      nudge.addEventListener('input', () => {
+        offset = Number(nudge.value)
+        readout.value = (offset > 0 ? '+' : '') + offset + ' s'
+        localStorage.setItem(key, offset)
+        place()
+      })
+    }
+  }
+
+  document.addEventListener('DOMContentLoaded', () => {
+    document.querySelectorAll('.lyricsbox.synced').forEach(enhanceSynced)
+    document.querySelectorAll('.lyricsbox.plain').forEach(enhancePlain)
+  })
+})()