fix(macapp): stop transcript autoscroll when the user reads earlier messages

[dennisonbertram]

Parent: #991

Summary

The transcript claims to autoscroll only while the user is at the bottom, but pinnedToBottom is initialized to true and never changes. Every appended item and every streamed assistant-text delta therefore calls scrollTo(bottomAnchor), pulling the viewport away from older content the user is reading.

Evidence

• macapp/Sources/GoCodeUI/ChatView.swift:88-114 — pin state is declared and every stream mutation calls scrollIfPinned.
• macapp/Sources/GoCodeUI/ChatView.swift:127-131 — the only pin-state consumer; no writer exists.
• Repository search finds no assignment that can make pinnedToBottom false.

Impact

A long-running response makes earlier transcript content effectively unreadable until streaming ends. This violates the view's own documented behavior and removes user control during the app's primary workflow.

Scope

• Track whether the bottom anchor is actually visible (or within a small semantic threshold).
• Pause autoscroll as soon as the user scrolls upward.
• Resume when the user returns to the bottom.
• Show a keyboard-accessible “Jump to Latest” affordance while new content arrives off-screen.
• Respect Reduce Motion for programmatic scrolling.

Out of scope

• Changing transcript row styling or storage.
• Replacing the whole transcript renderer.

Tests first

• Extract/test a small scroll-pin state reducer covering: initially pinned, user leaves bottom, streamed delta while unpinned, user returns to bottom.
• Production-reachability test proving the scroll-position signal writes the pin state.
• Manual smoke with a real streaming run: scroll upward mid-response and verify the viewport stays put; activate Jump to Latest and verify following resumes.

Acceptance criteria

• Streaming never changes the visible region while the user is reading above the bottom.
• Appended items and in-place deltas follow only when pinned.
• Jump to Latest is keyboard and VoiceOver operable.
• Reduced Motion avoids animated forced scrolling.
• swift test, Swift build, formatting, and strict format lint pass.

[dennisonbertram]

Repair scope for PR #1021 after independent review:

• Add red/green coverage for overlapping autoscroll completions so only the latest animation may re-enable geometry tracking.
• Complete the existing acceptance contract with a keyboard/VoiceOver-operable Jump to Latest affordance and Reduce Motion behavior.
• Preserve the scroll-pin reducer and production geometry wiring.

The live streaming installed-app smoke remains pending by explicit coordination until the separate native investigation returns. Current repair branch: codex/pr-1021-repair.

[dennisonbertram]

PR #1021 repair is pushed at 575b670938821f9db8b5bb096e3956753b0f0e25. #992 acceptance is reconciled with one cancellable, generation-checked autoscroll task, Reduce Motion handling, and accessible Jump to Latest. TranscriptScrollPinTests and full Swift (303 tests / 55 suites) pass. Installed-app scroll smokes remain under #1020.

[dennisonbertram]

PR #1021 exact head cec0a6654da96c7ef3ab812c489338f72c4fd6db now keys TranscriptView by the selected conversation ID, resetting pin/unseen/autoscroll state and cancelling pending scroll work when switching conversations. The reachability regression was red then green. Full Swift: 316 tests / 55 suites.