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.

apps/web/tests/composer-draft-scroll.e2e.ts

@@ -198,9 +198,12 @@ function measureComposer(page: Page): Promise<ComposerMetrics> {
  * @param top - metrics with the draft scrolled to its start.
  * @param bottom - metrics with the draft scrolled to its end.
  * @param trailingNewline - metrics with the trailing-newline draft scrolled to its end.
+ * @param pasted - metrics right after a long block was pasted at the draft's end.
  * @returns the golden body, without a trailing newline.
  */
-function renderGeometry(top: ComposerMetrics, bottom: ComposerMetrics, trailingNewline: ComposerMetrics): string {
+function renderGeometry(
+  top: ComposerMetrics, bottom: ComposerMetrics, trailingNewline: ComposerMetrics, pasted: ComposerMetrics,
+): string {
   return [
     '# Composer draft scrolling (14-line cap, two text layers, one scrollport)',
     '',
@@ -231,6 +234,14 @@ function renderGeometry(top: ComposerMetrics, bottom: ComposerMetrics, trailingN
     `- the draft's own last line is on screen: ${String(
       trailingNewline.lastLineOffset >= 0 && trailingNewline.lastLineOffset < trailingNewline.clientHeight,
     )}`,
+    '',
+    '## Right after pasting a long block at the end',
+    '',
+    `- the composer scrolled to the caret it left: ${String(pasted.scrollTop > 0)}`,
+    `- caret and glyphs stay level when the offset changes: ${String(pasted.gapShiftOnScroll === 0)}`,
+    `- the pasted block's last line is on screen: ${String(
+      pasted.lastLineOffset >= 0 && pasted.lastLineOffset < pasted.clientHeight,
+    )}`,
   ].join('\n').trimEnd()
 }
 
@@ -353,6 +364,37 @@ describe('web e2e: composer draft scrolling', () => {
     expect(tripwire.pageErrors).toEqual([])
   }, 60_000)
 
+  it('pasting a long block scrolls to the caret it leaves at the end', async () => {
+    onTestFailed(() => saveFailureShot(page, 'web-e2e-composer-draft-scroll-paste'))
+    // The composer suppresses the native paste — the machine owns the draft and
+    // the undo log — and restores the caret programmatically, which reveals
+    // nothing on its own: measured in chromium and WebKit, the view stayed
+    // where it was while the caret sat at the end of the pasted block. The
+    // restore now scrolls it into view, and this is the case that proves it.
+    const input = page.locator('textarea:enabled').first()
+    await input.fill('one short line')
+    await input.press('End')
+    // A real `paste` event carrying real clipboard data, dispatched at the
+    // textarea: the same event a Cmd-V delivers, and it runs the same handler.
+    await input.evaluate((el, text) => {
+      const data = new DataTransfer()
+      data.setData('text/plain', text)
+      el.dispatchEvent(new ClipboardEvent('paste', { clipboardData: data, bubbles: true, cancelable: true }))
+    }, `\n${DRAFT}`)
+    await expect.poll(async () => (await measureComposer(page)).overflows, { timeout: 10_000 }).toBe(true)
+    // The restore lands one frame after the machine commits the draft, so the
+    // box overflows before it moves; waiting on the offset is waiting for the
+    // behavior itself, and its absence fails this poll.
+    await expect.poll(async () => (await measureComposer(page)).scrollTop, { timeout: 10_000 }).toBeGreaterThan(0)
+    const metrics = await measureComposer(page)
+    // The caret is at the end of what was pasted, so the draft's last line is
+    // what has to be on screen.
+    expect(metrics.lastLineOffset).toBeGreaterThanOrEqual(0)
+    expect(metrics.lastLineOffset).toBeLessThan(metrics.clientHeight)
+    expect(metrics.gapShiftOnScroll).toBe(0)
+    expect(tripwire.pageErrors).toEqual([])
+  }, 60_000)
+
   it('a draft ending in a newline scrolls to its true end, not a line above it', async () => {
     onTestFailed(() => saveFailureShot(page, 'web-e2e-composer-draft-scroll-trailing-newline'))
     // The layers reserve a final line box on different terms, so this shape is
@@ -399,7 +441,19 @@ describe('web e2e: composer draft scrolling', () => {
       return m.scrollTop === m.scrollMax
     }, { timeout: 10_000 }).toBe(true)
     const trailingNewline = await measureComposer(page)
-    await compareOrRefreshGolden(GEOMETRY_EXPECTED, renderGeometry(top, bottom, trailingNewline), MODE)
+    // The paste path, measured the way a user meets it: a short draft, the
+    // caret at its end, one long block pasted in.
+    await input.fill('one short line')
+    await input.press('End')
+    await input.evaluate((el, text) => {
+      const data = new DataTransfer()
+      data.setData('text/plain', text)
+      el.dispatchEvent(new ClipboardEvent('paste', { clipboardData: data, bubbles: true, cancelable: true }))
+    }, `\n${DRAFT}`)
+    await expect.poll(async () => (await measureComposer(page)).overflows, { timeout: 10_000 }).toBe(true)
+    await expect.poll(async () => (await measureComposer(page)).scrollTop, { timeout: 10_000 }).toBeGreaterThan(0)
+    const pasted = await measureComposer(page)
+    await compareOrRefreshGolden(GEOMETRY_EXPECTED, renderGeometry(top, bottom, trailingNewline, pasted), MODE)
     expect(tripwire.pageErrors).toEqual([])
   }, 60_000)