The room also measures the recording. `lyrics_vocal_activity` is the
desktop's coarse vocal-band envelope — the centre channel band-passed to
180 Hz – 7.5 kHz, one value per 100 ms, read on a blocking thread — and it
draws as a strip under the head, so the singing ahead is visible while the
thumb waits for it. **Estimate the timing** lays every line across the
stretches the envelope says are sung, weighted by line length, through the
desktop's own generator (`src/lib/lyric-sync.js`, shared and unit-tested):
never an even spread over the duration, and a refusal rather than a guess
when the measurement shows nothing usable. The result is labelled an
estimate everywhere it shows — the hint under the line, the Save button —
the cursor goes back to the top so the taps correct it from there, and one
more tap on the button drops the whole thing back to an empty set (a rough
fill stamps every line at once, so Undo has nothing left to take back).
A streamed track has no finished file to measure, so the button says there
is no estimate for that one — the same rule the reduced-vocals copy follows,
and the same whole-file decode behind both.