Keep the macOS cursor overlay alive at screen edges

[enaboapps]

Closes #510

What changed

• Preserve existing half-open monitor ownership when a macOS display contains the cursor.
• Fall back deterministically to the nearest display when the pointer is on an outer edge, corner, or display gap.
• Keep the marker centered on the real cursor so off-screen portions remain clipped rather than visually offset.
• Add pure Rust coverage for every outer edge and corner, shared boundaries, negative-coordinate layouts, gaps, ties, and an empty display list.

Root cause

An exact upper or right boundary coordinate matched no NSScreen. The resulting render error terminated the cursor-overlay worker, so moving away from the edge could not restore it.

Validation

• npm run lint
• npm test
• npm run build
• cargo fmt --manifest-path src-tauri/Cargo.toml --check
• cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets -- -D warnings
• cargo test --manifest-path src-tauri/Cargo.toml
• npm run tauri build -- --bundles app,dmg --no-sign
• npm run macos:run; stable certificate requirement verified and signed Preview launched

The live pointer edge/corner smoke check is pending because the Mac desktop was locked when Computer Use attempted to inspect the launched app.

[enaboapps]

@greptileai please review the latest commit 5bc3dbf and provide a full score.

[greptile-apps]

Greptile Summary

The PR keeps the macOS cursor overlay worker alive when the pointer does not belong to a display’s half-open frame.

• Preserves existing half-open ownership for points contained by a display.
• Selects the nearest display for outer boundaries, corners, and geometric gaps.
• Adds focused unit coverage for boundaries, negative coordinates, ties, gaps, and an empty display list.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete changed-code failure identified.

The new lookup preserves contained-point ownership, deterministically resolves otherwise unmatched coordinates, safely returns None for an empty screen list, and is compatible with the repository’s pinned Rust toolchain.

Important Files Changed

Filename	Overview
cross-platform/src-tauri/src/overlay_macos.rs	Replaces strict display containment with containment-or-nearest selection and adds comprehensive pure geometry tests; no actionable defect was identified.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Read macOS cursor position] --> B{Point inside a half-open screen frame?}
    B -- Yes --> C[Use containing screen]
    B -- No --> D{Any screens available?}
    D -- No --> E[Return resolution error]
    D -- Yes --> F[Compute squared distance to each frame]
    F --> G[Use nearest screen; first screen wins ties]
    C --> H[Render and position overlay]
    G --> H

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