[Bug] With autoScrollEnabled: false, viewport drifts by N lines per N new streamed lines once user prompt scrolls off-screen

Description

With autoScrollEnabled: false, the viewport stays anchored correctly while the user's prompt is still visible on screen. But once the user scrolls far enough that the prompt leaves the viewport, the displayed content drifts downward in lockstep with the streaming output — gaining one line of offset per new line generated — instead of staying pinned to the visible content.

Steps to Reproduce

1. Set "autoScrollEnabled": false in ~/.claude/settings.json.
2. Send a prompt that produces a long streaming response (≥100 lines).
3. While Claude is still generating, scroll down so that the user prompt is no longer visible — e.g. center "Line 14" of the response in the viewport.
4. Observe: as new lines stream in, the line in the middle of the screen advances (Line 14 → 15 → 16 → ...) instead of staying on Line 14.

Expected Behavior

With auto-scroll disabled, the viewport should stay pinned to whatever content is currently visible, regardless of whether the prompt is in frame.

Actual Behavior

Viewport drifts downward by N lines for every N new lines streamed, once the prompt is off-screen.

Environment

• Claude Code: 2.1.118
• macOS: 26.3.1
• App: Claude Code desktop app (fresh launch, no project-level settings)
• ~/.claude/settings.json has "autoScrollEnabled": false; no override in settings.local.json