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A composer draft past the 14-line cap could not be scrolled: the caret and the selection moved, but the words stayed frozen at line 1, so the tail of anything longer than the cap was unreachable while writing it. The composer paints its text in two stacked layers. The textarea owns the value, the selection and the caret but renders its own glyphs transparent; every visible character is painted by the decoration backdrop beneath it, which also carries the claim-token highlight, the chips and the ghost hint. The backdrop is `inset: 0; overflow: hidden` — clipped, not scrolled — and nothing linked its offset to the textarea's. Below the cap both layers rest at 0, which is why the defect hid behind every short-draft screenshot and fixture. InputBar now mirrors the textarea's scrollTop onto the backdrop, from a `scroll` listener (every gesture and every caret-driven scroll) and from a layout effect keyed on the committed draft (an edit reflows both layers without necessarily firing a scroll event). Scrolling is layout, so jsdom cannot show this: the unit spec stubs both offsets and proves the mirroring paths run, while a new browser scenario measures the user-visible fact against the built client with a DOM Range over the backdrop's own text — after a wheel gesture over a 40-line draft the last line is on screen and the first has scrolled out. Confirmed both directions: with the mirroring reverted and the packages rebuilt, the golden reads `last draft line is on screen: false` while `textarea moved: true`.