Sync macOS scroll containers with the engine

[dogukani]

Problem

On macOS, NSScrollView owns a scroll container's position but never reported it to the engine. Tree::scrollTop stayed 0, so:

• a VirtualList kept its first row window, and rows past #16 were blank;
• scroll listeners never fired, because Tree::setScrollTop is what dispatches them.

Change

• GeaFlippedClipView commits each settled bounds change with Tree::setScrollTop, as the iOS container does. It does not commit while rubber-banding. A change made during sync is committed after the walk.
• Children of a scroll container are placed in document space (node.layout.y - scrollTop), as on iOS, so they are not offset twice.

Verification

• targets/macos/test/native-scroll-commit.test.mjs pins the wiring. npm test passes (14 files).
• A local check (not part of this PR) scrolled the virtual-list example through the clip view programmatically:
  • Before this change, treeScrollTop=0 and rows Sync macOS scroll containers with the engine #1–#16 were shown.
  • After it, treeScrollTop matched y = 2140, 214000, 534498 and 0, and the matching rows were shown.
• Manual trackpad test: rows past #16 appear and update during movement, and the top returns to Sync macOS scroll containers with the engine #1. Captured frames showed no blank or mismatched rows.
• Tic Tac Toe builds and passes GEA_MACOS_VERIFY_ONCE.
• No iOS files changed. The iOS simulator run of the same example passed separately.

Related

An always-shown (legacy) scroller hides the right edge of scroll-container content and lets it scroll sideways, and the scroller knob cannot be dragged. Both happen on main as well. #2 addresses them.

Dependencies

macOS Virtual List scrolling needs this PR and geastack/core#57, geastack/compiler#1 and geastack/examples#1, which are merged. The example only refreshes its rows on the scroll event those PRs add.

Not covered

• overflow: scroll Views use the same path but were not exercised separately.

Summary by CodeRabbit

• Bug Fixes
  • macOS scroll views now keep their displayed position synchronized with the document’s scroll position, including when the view’s bounds change.
  • Scrolled content is positioned relative to the document, and scroll offsets outside the valid range are ignored or clamped.
  • Scrollable documents now respect the visible width of their view.

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📝 Walkthrough

Walkthrough

The macOS renderer now commits valid AppKit scroll offsets to tree state. It defers commits during renderer sync and uses scrollTop to position scroll-view children.

Changes

macOS Scroll Synchronization

Layer / File(s)	Summary
Native scroll state and layout packages/geastack-apple/targets/macos/main/macos_renderer.mm, packages/geastack-apple/targets/macos/test/native-scroll-commit.test.mjs	The clip view commits valid offsets to its tree node and defers commits during renderer sync. The renderer positions scroll-view children using scrollTop. The source-level test checks these behaviors and their wiring.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix

Merge Risk: 🔵 Low · up to d0b8d

With a visible legacy scroller, right-aligned content may be clipped. Confirm or fix the document-width handling before merging.

Security Architecture Review

Security architecture risk: 🔵 Low · up to d0b8d

Scroll offsets are checked before they reach the engine, and the visible change is confined to macOS scrolling. A deferred update may nevertheless outlive the view that produced it; no security exploit is established.

Retained concerns

• Low · reliability · inferred: A bounds notification deferred during sync can commit using a detached clip view's node ID after its scroll view has been replaced or removed. The node-count check does not establish current view ownership, so stale scroll state or an event for the wrong lifecycle state is possible.

Security review details

Security Blast Radius

• inferred — The visible input-to-state path is native macOS scrolling for virtual lists and overflow-scroll views. The inspected changes do not establish a network-facing entrypoint or a privileged sink.

Trust Boundaries and Controls

• inferred — The relevant boundary is AppKit-owned viewport position entering engine-owned node state. Range and node-count checks constrain values, but a deferred commit has no current-owner check. No attacker path to a sensitive outcome is established.

Resilience and Maintainability Implications

• inferred — Removal unregisters notification observation when the clip view deallocates, but an already queued block retains that view and can run after renderer cleanup. Multiple notifications can also queue multiple commits; whether the tree suppresses unchanged events is unverified.

Hardening Proposals

• proposed — Before a deferred commit, verify that the clip view still belongs to the node's current scroll view; invalidate ownership on replacement or teardown, and exercise that transition in a runtime test.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (1 skipped: 1 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: synchronizing macOS scroll containers with the engine.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (1 skipped: 1 unsupported.)

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Inline comments:
In `@packages/geastack-apple/targets/macos/main/macos_renderer.mm`:
- Line 1585: Update the width calculation in applyScrollContentSize so a visible
legacy scroller cannot narrow the document below node.layout.width; preserve
engine-layout coordinates used by syncRecursive so right-aligned children remain
reachable.

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📒 Files selected for processing (5)

• packages/geastack-apple/targets/macos/README.md
• packages/geastack-apple/targets/macos/main/macos_main.mm
• packages/geastack-apple/targets/macos/main/macos_renderer.h
• packages/geastack-apple/targets/macos/main/macos_renderer.mm
• packages/geastack-apple/targets/macos/test/native-scroll-commit.test.mjs

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