fix(web): bind the composer's caret to its glyphs with one scrollport

The composer paints its draft in two layers — the textarea owns the value,
the selection and the caret, the backdrop paints every visible glyph — and
they had one scroll offset each, kept equal by a `scroll` listener. That
holds at rest and not in motion: a wheel gesture scrolls the textarea on the
compositor, the listener runs afterwards, and for those frames the caret sits
at the new offset with the words at the old one. Measured on a harness of the
same geometry, a 200px offset change separates caret from glyphs by ~200px
(chromium 203, firefox 202, WebKit 203) until a later frame — the caret
flying out of its own text when a user swipes a long draft quickly.

Both layers now ride one scrollport: `[data-input-scroll]` carries the
14-line cap, the auto-grow stack inside it is as tall as the whole draft, and
the textarea holds no scrollable overflow of its own. The browser applies one
offset to both layers in the same frame, so the coupling is structural rather
than maintained. The backdrop's trailing-line sentinel and the cross-engine
wrap-width premise go with the mirror: the layers now share a containing
block, which closes the WebKit 768-against-776 gap by construction.

packages/client/ui-conversation/tests/input-bar.spec.tsx

@@ -239,7 +239,7 @@ describe('running and lock semantics (queue cut 1)', () => {
     expect((textarea).value).toBe('typed')
   })
 
-  it('wheel over a non-overflowing textarea forwards to the conversation host', () => {
+  it('wheel over a non-overflowing draft forwards to the conversation host', () => {
     const host = document.createElement('div')
     host.setAttribute('data-conversation-scroll', '')
     Object.defineProperty(host, 'scrollTop', { value: 40, writable: true, configurable: true })
@@ -255,17 +255,18 @@ describe('running and lock semantics (queue cut 1)', () => {
     }
   })
 
-  it('wheel chains: long drafts scroll inside the textarea until each edge, then the host', () => {
+  it('wheel chains: long drafts scroll inside the draft scrollport until each edge, then the host', () => {
     const host = document.createElement('div')
     host.setAttribute('data-conversation-scroll', '')
     Object.defineProperty(host, 'scrollTop', { value: 40, writable: true, configurable: true })
     const { view, textarea } = bench()
     host.appendChild(view.container)
     document.body.appendChild(host)
-    Object.defineProperty(textarea, 'clientHeight', { value: 100, configurable: true })
-    Object.defineProperty(textarea, 'scrollHeight', { value: 400, configurable: true })
+    const scrollport = view.container.querySelector<HTMLElement>('[data-input-scroll]')!
+    Object.defineProperty(scrollport, 'clientHeight', { value: 100, configurable: true })
+    Object.defineProperty(scrollport, 'scrollHeight', { value: 400, configurable: true })
     let scrollTop = 150
-    Object.defineProperty(textarea, 'scrollTop', {
+    Object.defineProperty(scrollport, 'scrollTop', {
       configurable: true,
       get: () => scrollTop,
       set: (value: number) => { scrollTop = value },
@@ -289,35 +290,20 @@ describe('running and lock semantics (queue cut 1)', () => {
     }
   })
 
-  it('the decoration backdrop tracks the textarea offset (it paints every visible glyph)', () => {
+  it('the caret layer and the glyph layer ride one scrollport', () => {
     const { view, textarea } = bench({ draft: 'line\n'.repeat(40) })
+    const scroll = view.container.querySelector<HTMLElement>('[data-input-scroll]')!
     const backdrop = view.container.querySelector<HTMLElement>('[data-input-backdrop]')!
-    Object.defineProperty(backdrop, 'scrollTop', { value: 0, writable: true, configurable: true })
-    Object.defineProperty(textarea, 'scrollTop', { value: 0, writable: true, configurable: true })
-    // A scrolled draft: the textarea moves, the clipped backdrop must follow.
-    textarea.scrollTop = 120
-    fireEvent.scroll(textarea)
-    expect(backdrop.scrollTop).toBe(120)
-    // Every later move tracks too, including back to the top — a one-shot
-    // mirror would leave the glyphs parked at the first offset it saw.
-    textarea.scrollTop = 0
-    fireEvent.scroll(textarea)
-    expect(backdrop.scrollTop).toBe(0)
-  })
-
-  it('the backdrop carries the trailing-line sentinel that keeps its extent equal to the textarea', () => {
-    // jsdom has no layout, so the HEIGHTS this protects cannot be asserted here
-    // (the browser scenario owns that); what is checkable is that the backdrop's
-    // text is the draft plus exactly one newline. A textarea reserves a line box
-    // after a final newline and `pre-wrap` collapses one, so without the
-    // sentinel a draft ending in a newline leaves the backdrop a line short and
-    // the mirrored offset clamps.
-    const withNewline = bench({ draft: 'alpha\nbeta\n' })
-    const backdrop = withNewline.view.container.querySelector<HTMLElement>('[data-input-backdrop]')!
-    expect(backdrop.textContent).toBe('alpha\nbeta\n\n')
-    const withoutNewline = bench({ draft: 'alpha\nbeta' })
-    const plain = withoutNewline.view.container.querySelector<HTMLElement>('[data-input-backdrop]')!
-    expect(plain.textContent).toBe('alpha\nbeta\n')
+    // The caret is the textarea's and every visible glyph is the backdrop's, so
+    // one box has to carry both or an offset can exist in one and not the other.
+    // jsdom has no layout — the browser scenario owns the geometry; what is
+    // checkable here is that there is exactly one scrolling box and it holds
+    // both layers.
+    expect(scroll.contains(textarea)).toBe(true)
+    expect(scroll.contains(backdrop)).toBe(true)
+    // The glyph layer carries the draft and nothing else: with one scrollport
+    // it no longer pads its own height to match a second box's scroll extent.
+    expect(backdrop.textContent).toBe('line\n'.repeat(40))
   })
 
   it('disabled state shows the unavailable placeholder; custom placeholder wins', () => {