fix: pr-bug-scan validated finding from #6319

[buf0-bot]

Automated fix-PR from pr-bug-scan for parent #6319.

Fixer status: FIXED_WITH_CODE_PROOF
Summary: Gated pinned-viewport re-pin on output writes to only correct downward auto-follow drift, not chase a stale absolute target down after a scrollback-cap trim.
Blocked proof link: src/renderer/src/lib/pane-manager/terminal-scroll-intent.ts:220-221 (scrollToLine(X) on a trim-decremented viewport) is now blocked: enforceTerminalWriteScrollIntent skips the re-pin when fromOutputWrite and current.viewportY < targetY
Typecheck: skipped

Proof (from validator)

• C1 — proof_type: code_analysis
  • trigger: User wheels up in a visible normal-buffer terminal (sets pinnedViewport via markTerminalPinnedViewport) and reads scrollback while sustained PTY output streams in after the scrollback buffer is full (at its line cap).
  • observable: While pinned reading scrollback under heavy streaming at the scrollback cap, the visible content drifts/jumps downward toward the live bottom on every write batch by the number of pruned lines, instead of remaining on the pinned content — eventually losing the user's reading position.
  • path:
    • src/renderer/src/lib/pane-manager/terminal-scroll-intent.ts:onWheel (deltaY<0) calls markTerminalPinnedViewport -> stored intent kind='pinnedViewport'
    • src/renderer/src/lib/pane-manager/pane-terminal-output-scheduler.ts:631 writeForegroundTerminalChunkWithIntent calls captureTerminalWriteScrollIntent(terminal) -> snapshot.viewportY = X (absolute ydisp before write), kind='pinnedViewport'
    • node_modules/@xterm/xterm/src/common/services/BufferService.ts:83 willBufferBeTrimmed = buffer.lines.isFull (true at cap); line 106-108: with isUserScrolling true, buffer.ydisp = Math.max(buffer.ydisp - 1, 0) per scrolled line while ybase stays at cap — keeps displayed content stable
    • src/renderer/src/lib/pane-manager/pane-terminal-output-scheduler.ts:635 onParsed -> enforceTerminalWriteScrollIntent(terminal, snapshot)
    • src/renderer/src/lib/pane-manager/terminal-scroll-intent.ts:219 targetY = clampViewportY(snapshot.viewportY=X, current.baseY=cap) = X; line 220-221 current.viewportY (X-N) !== X -> terminal.scrollToLine(X)
    • node_modules/@xterm/xterm/src/common/CoreTerminal.ts:218 scrollToLine issues scrollLines(+N), moving the viewport down by the pruned line count; positive disp keeps isUserScrolling true (BufferService.ts:141) so it repeats each batch

Reproduction

pnpm exec vitest run --config config/vitest.config.ts src/renderer/src/lib/pane-manager/terminal-scroll-intent.test.ts -t "does not chase a stale absolute viewport down"

• before fix: scrollToLine called with the stale absolute line (viewport dragged toward live bottom)
• after fix: scrollToLine not called; viewport stays on trim-stabilized pinned content

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Generated by pr-bug-scan (proof-machine architecture). Human approval required before merge.