fix(web): scroll to the caret after an edit the composer performs itself

Pasting a long block left the view where it was while the caret sat at the
end of what was pasted. Paste, ctrl/meta-Enter newline and cut all suppress
the native edit — the machine owns the draft and the undo log — and restore
the caret with `setSelectionRange`, which reveals nothing: measured in
chromium and WebKit, before this branch as well as on it. Firefox happened to
reveal it, in the old geometry only.

The three restores now share one helper that measures the caret against the
hidden mirror — same draft, same metrics, same wrap width, so a Range
collapsed at the caret's index reports where the caret is without a caret API
— and scrolls the scrollport the minimum that brings the line inside, which
is what the browser does for typing. One scrollport is what makes this
possible at all: the reveal is finally a single offset to move.

Also from review: the composer's own focus() on unlock and session switch
passes preventScroll, so a session switch cannot move the transcript through
the taller textarea's reveal chain.

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

@@ -4,7 +4,7 @@
 // semantics (input stays free; primary turns stop), the machine pending lock,
 // decoration backdrop, error/notice strips, and the focus-keeping mousedown.
 
-import { afterEach, describe, expect, it, vi } from 'vitest'
+import { afterEach, describe, expect, it, onTestFinished, vi } from 'vitest'
 import { act, cleanup, fireEvent, render } from '@testing-library/react'
 import { bindSnapshotSelector } from '@deepseek-ai/dsh-client-web-react'
 import { createSnapshotStore } from '@deepseek-ai/dsh-client-runtime/client'
@@ -18,6 +18,13 @@ import { zh } from '../src/client/locales.ts'
 
 afterEach(cleanup)
 
+// jsdom implements no Range geometry at all — `Range.prototype.getBoundingClientRect`
+// is absent — and the composer measures the caret with one when it restores the
+// selection after an edit it performed itself. Every case here runs against a
+// zero rect; the reveal case below substitutes its own and restores this one.
+const ZERO_RECT = (): DOMRect => ({ top: 0, bottom: 0 }) as DOMRect
+Range.prototype.getBoundingClientRect = ZERO_RECT
+
 const SCTX = {} as ClientContext
 const SID = 's1' as SessionId
 
@@ -296,9 +303,9 @@ describe('running and lock semantics (queue cut 1)', () => {
     const backdrop = view.container.querySelector<HTMLElement>('[data-input-backdrop]')!
     // 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.
+    // jsdom has no layout and loads no stylesheet — which box scrolls is the
+    // browser scenario's to assert; what is checkable here is that the
+    // scrollport element 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
@@ -306,6 +313,41 @@ describe('running and lock semantics (queue cut 1)', () => {
     expect(backdrop.textContent).toBe('line\n'.repeat(40))
   })
 
+  it('an edit the composer performs itself scrolls the caret back into view', async () => {
+    // Paste, ctrl-Enter newline and cut suppress the native edit, so no engine
+    // reveals the caret for them. jsdom has no layout: the rects are stubbed,
+    // and what is asserted is the arithmetic — minimal scroll, in both
+    // directions, and nothing at all for a caret already inside the box.
+    const { view, textarea } = bench({ draft: 'line\n'.repeat(40) })
+    const scroll = view.container.querySelector<HTMLElement>('[data-input-scroll]')!
+    const mirror = view.container.querySelector<HTMLElement>('[data-input-mirror]')!
+    expect(mirror.firstChild).toBeInstanceOf(Text)
+    scroll.getBoundingClientRect = () => ({ top: 100, bottom: 436 }) as DOMRect
+    Object.defineProperty(scroll, 'scrollTop', { value: 0, writable: true, configurable: true })
+    onTestFinished(() => { Range.prototype.getBoundingClientRect = ZERO_RECT })
+    const caretAt = (top: number): void => {
+      Range.prototype.getBoundingClientRect = () => ({ top, bottom: top + 24 }) as DOMRect
+    }
+    const settle = async (): Promise<void> => {
+      await act(async () => { await new Promise((resolve) => { requestAnimationFrame(() => { resolve(null) }) }) })
+    }
+    // Pasted text lands below the fold: scroll down by exactly the overshoot.
+    caretAt(500)
+    fireEvent.paste(textarea, { clipboardData: { getData: () => 'pasted' } })
+    await settle()
+    expect(scroll.scrollTop).toBe(88) // 524 - 436
+    // A caret already inside the box does not move it.
+    caretAt(200)
+    fireEvent.paste(textarea, { clipboardData: { getData: () => 'more' } })
+    await settle()
+    expect(scroll.scrollTop).toBe(88)
+    // Above the fold (a cut can leave it there): scroll back up.
+    caretAt(60)
+    fireEvent.paste(textarea, { clipboardData: { getData: () => 'again' } })
+    await settle()
+    expect(scroll.scrollTop).toBe(48) // 88 - (100 - 60)
+  })
+
   it('disabled state shows the unavailable placeholder; custom placeholder wins', () => {
     const { textarea } = bench({ disabled: true })
     expect(textarea.placeholder).toBe('会话不可用')