Fix Ghostty viewport sync scroll jitter

LA
8mo ago
+682-35

Summary

Restart from current main and rebuild the Ghostty viewport sync fix with a fresh red/green pair.

  • add a pure viewport sync helper plus regression coverage for anchor preservation, explicit scroll consumption, stale passive updates, and AppKit row mapping
  • keep Ghostty top-visible-row state separate from AppKit bottom-origin scroll distance
  • use accepted scrollbar state and stored top-row anchors in GhosttyTerminalView so plain scrolling and passive updates stop fighting each other

Verification

  • ./tests/test_ghostty_viewport_sync_logic.sh
  • xcodebuild -project GhosttyTabs.xcodeproj -scheme cmux-unit -destination "platform=macOS" -derivedDataPath /tmp/cmux-task-scroll-jank-reset-green4 -only-testing:cmuxTests/GhosttyScrollbarSyncPlanTests test
  • ./scripts/reload.sh --tag task-scroll-jank-reset

Summary by cubic

Fixes scroll jitter in Ghostty by separating explicit user scrolls from passive output updates and anchoring the top visible row. Adds a stateless viewport sync planner to stabilize scrollbar-to-viewport mapping and prevent rubber-banding.

  • Bug Fixes

    • Marks explicit viewport changes on wheel and user-initiated binding actions, and treats output updates as passive to avoid conflicts.
    • Stores the top-visible-row anchor and reconciles passive scrollbar snapshots to that anchor; uses AppKit’s row-from-bottom mapping for accurate targets.
    • Ignores regressive/stale passive snapshots while reviewing scrollback to prevent anchor loss and jitter.

  • Refactors

    • Extracted sync logic into GhosttyViewportSync.swift with small, pure helpers used by GhosttySurfaceScrollView/GhosttyTerminalView.
    • Added GhosttyScrollbarSyncPlanTests.swift and a lightweight CLI test (tests/test_ghostty_viewport_sync_logic.*); wired tests into the Xcode project.

Summary by CodeRabbit

  • New Features

    • Enhanced scrollbar and viewport synchronization for improved scrolling behavior and consistency.

  • Refactor

    • Updated internal scrolling logic to use notification-driven updates and anchor-based viewport tracking for more reliable scroll position management.

  • Tests

    • Added comprehensive test coverage for viewport synchronization behavior.