feat(client): echo a DNA band hover onto its feed row

Hovering a band on the DNA bar now lights the row it names below — a message
band its exact feed row, a tool-schema band the tools group row, the system
band the system row — the reverse of the row-hover-to-band link, so the two
surfaces read as one surface. The echoed row also scrolls into view, confined
to the nearest scrollable ancestor's own scrollport (the capped column): the
row only has to reach the scrollport's visible slice, since a capped column
usually extends below the page fold and a page scroll would drag the pointer
off the pinned band mid-scrub. Clicking a band still lands on the row and
expands it.

src/client/components/browser.tsx

@@ -325,6 +325,32 @@ export function makeContextBrowser(
       && (rowDnaKey === 'tools' ? view.header !== null
         : rowDnaKey === 'system' ? view.system !== null
           : dnaByKey.has(rowDnaKey)) ? rowDnaKey : null
+    // A band hover brings its echoed row into view — within the nearest scrollable ancestor's own scrollport
+    // (the capped column) and nothing above it: the pointer rests on the pinned bar, and a page scroll would
+    // drag it off the band mid-scrub.
+    useLayoutEffect(() => {
+      if (liveDnaKey === null) return
+      /* v8 ignore next 1 -- liveDnaKey non-null implies a mounted card: the ref always resolves. */
+      const row = rootRef.current?.querySelector('.lc-br-elem-echo') ?? null
+      /* v8 ignore next 1 -- a live band names a rendered item, so its echoed row always exists. */
+      if (row === null) return
+      let scroller: HTMLElement | null = row.parentElement
+      while (scroller !== null && scroller.scrollHeight <= scroller.clientHeight) {
+        scroller = scroller.parentElement
+      }
+      /* v8 ignore next 1 -- jsdom lays out nothing: no ancestor ever overflows. */
+      if (scroller === null) return
+      /* v8 ignore start -- real layout only: jsdom measures no boxes. */
+      const rb = row.getBoundingClientRect()
+      const sb = scroller.getBoundingClientRect()
+      // The row only has to reach the scrollport's VISIBLE slice: a capped column usually extends below the
+      // page fold, so its own box bottom is no visibility boundary.
+      const visTop = Math.max(sb.top, 0)
+      const visBottom = Math.min(sb.bottom, window.innerHeight)
+      if (rb.top < visTop) scroller.scrollTop -= visTop - rb.top
+      else if (rb.bottom > visBottom) scroller.scrollTop += rb.bottom - visBottom
+      /* v8 ignore stop */
+    }, [liveDnaKey])
     const pickDna = (key: string): void => {
       const it = dnaByKey.get(key)
       /* v8 ignore next 1 -- the bar only reports keys of the parts it was handed; defensive. */
@@ -407,7 +433,9 @@ export function makeContextBrowser(
 
     /** Expandable element row; `err` carries the red run-state dot right after the chevron so a failed result scans while collapsed.
      * `opts.open` overrides the `openElem === key` derivation for rows whose key set is wider (the flat feed's tools group);
-     * `opts.onHover` lets a DNA feed row light its own band on the composition bar. */
+     * `opts.onHover` lets a DNA feed row light its own band on the composition bar. The reverse echo — a DNA band
+     * hover lighting its row — derives straight from the band key: a node or the system prompt matches its own row,
+     * a tool-schema band matches the tools group row. */
     const elemRow = (
       key: string, tag: ReactNode | null, preview: string,
       tokens: number, time: number | undefined, body: ReactNode,
@@ -417,8 +445,10 @@ export function makeContextBrowser(
     ) => {
       const open = opts?.open ?? openElem === key
       const hover = opts?.onHover
+      const echo = dna && liveDnaKey !== null
+        && (liveDnaKey === key || (key === 'tools' && liveDnaKey.startsWith('tool:')))
       return (
-        <div key={key} className={'lc-br-elem' + (open ? ' lc-br-elem-on' : '')}>
+        <div key={key} className={'lc-br-elem' + (open ? ' lc-br-elem-on' : '') + (echo ? ' lc-br-elem-echo' : '')}>
           <button
             type="button"
             className="lc-br-elem-row hover:bg-(--dsw-alias-interactive-bg-hover)"

src/client/styles/browser.css

@@ -88,6 +88,8 @@
 .lc-br-time { flex: none; color: var(--dsw-alias-label-secondary); font-size: 11px; white-space: nowrap; }
 /* A PENDING stamp (a live-tail item no request carries yet) is an estimate; its tooltip names the not-yet-sent step. */
 .lc-br-time-pending { text-decoration: underline dotted; text-underline-offset: 2px; }
+/* A DNA band hover echoes onto its feed row: the row's button wears the hover wash while the pointer is on the bar. */
+.lc-br-elem-echo > .lc-br-elem-row { background-color: var(--dsw-alias-interactive-bg-hover); }
 /* Call-hit tally on a tool-definition row. */
 .lc-br-hits { flex: none; color: var(--dsw-alias-label-secondary); font-size: 11px; }
 .lc-br-content { padding: 2px 6px 8px 26px; display: flex; flex-direction: column; gap: 6px; }

tests/client/components/browserDna.spec.ts

@@ -326,6 +326,24 @@ describe('ContextBrowser DNA mode and the open-category bar pin', () => {
     await m.unmount()
   })
 
+  test('hovering a DNA band echoes onto its feed row; a tool-schema band lights the tools group row', async () => {
+    const m = await mount(h(Browser, props({ data: dnaData, headers: dnaHeaders })))
+    await click(dnaButton(m))
+    // Band n3 (index 4) is feed row index 2 (the feed runs newest-first).
+    await hover(bands(m)[4])
+    assert.ok(flatRows(m)[2].className.includes('lc-br-elem-echo'))
+    assert.ok(flatRows(m).every((r, i) => i === 2 || !r.className.includes('lc-br-elem-echo')))
+    await unhover(bands(m)[4])
+    assert.ok(flatRows(m).every(r => !r.className.includes('lc-br-elem-echo')))
+    // A tool-schema band echoes onto the tools group row; the system band onto the system row.
+    await hover(bands(m)[1])
+    assert.ok(flatRow(m, 'Tool Schemas').closest('.lc-br-elem')?.className.includes('lc-br-elem-echo'))
+    await unhover(bands(m)[1])
+    await hover(bands(m)[0])
+    assert.ok(flatRow(m, 'System Prompt').closest('.lc-br-elem')?.className.includes('lc-br-elem-echo'))
+    await m.unmount()
+  })
+
   test('a tiny item stays a hoverable filament (the minBand floor) while the tooltip reports the true share', async () => {
     const data = tl({
       current: { system: 0, tools: 0, user: 2, inject: 0, skill: 0, assistant: 998, tool: 0, total: 1000 },