Empirically diagnosed via live browser measurement: Source Sans 3's
font box is 22px at 16px, but lh-input's 1.25 leading gives a 20px line
box - the font's content area sticks 1px below the line, Chromium
counts it in scrollHeight, and the zero-padding field-sizing textarea
turned it into a permanent 1px scroll: the bar showed EMPTY and at
every expansion size below max-h (owner report). The composer and dock
fields never showed it because their py-2 absorbs the surplus
implicitly. Fix: py-0.5 on the reading field - the same absorption,
line-height untouched (the caret law, typography round 2), and the
max-h-40 cap still scrolls honestly past 160px of text.
