[FEATURE]: Auto-scroll to top of new assistant response instead of bottom

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Feature request

When the assistant finishes streaming a response that's longer than the viewport, the scroll position stays at the bottom of that response, forcing me to scroll up manually to read it from the top.

Please add either:

1. A preference toggle in Desktop → General (or similar) to auto-scroll to the top of the newest response when it finishes rendering, OR
2. A dedicated "Jump to Answer Start" button/shortcut as previously requested in #9794.

Related and prior work

• #9794 — [FEATURE]: Add 'Jump to Answer Start' Button. Closed as COMPLETED but based on the last automated comment, it appears to have been auto-closed after 90 days of inactivity rather than actually implemented. The feature is not present in the desktop app v1.18.14.
• #3337 — Navigation Shortcut to Jump Between User Messages. Also closed similarly. Related but different (jumps between user messages, not to top of assistant response).
• #5783 — Scroll down UI button when !bottom. Already implemented (Jump to Latest button exists at the bottom of long conversations). This request is the mirror-image: a way to jump to the top of the newest response.
• #9227 — Bookmarks in thread. Alternative approach, not equivalent.

None of the above solve the specific problem: I want reading a new response to start from the top of that response, ideally automatically.

Desired behavior

Preferred approach (automatic):

• New toggle: "Scroll to top of new response" (default off, to preserve current behavior).
• When enabled: after the assistant finishes streaming, viewport scrolls so the first line of the newest response is at the top.

Acceptable fallback (manual):

• Reintroduce the "Jump to Answer Start" button from #9794 at the top of each assistant response, OR
• Add a keyboard shortcut like Cmd+Home or Alt+↑ to jump to the beginning of the latest assistant message.

Why this matters

• Reading top-to-bottom is the natural flow; landing at the bottom of a long response is disorienting.
• On mixed content (explanation + code + tables), the manual scroll adds friction on every turn.
• Users who prefer the current auto-scroll-to-bottom behavior can leave the toggle off.

Environment

• OpenCode Desktop 1.18.14
• macOS 26.5.2
• Verified no scroll-related option exists under Settings → Desktop → General.
• Verified no "jump to previous message" shortcut exists under Settings → Desktop → Shortcuts.