Respect WindowBuilder vsync in render surface configuration

[vmarcella]

Summary

Fixes vsync configuration being effectively ignored in some cases by ensuring
the requested present mode is respected end-to-end and that “vsync off”
fallbacks prefer true non-vsync modes when available.

Related Issues

• Resolves [Bug] WindowBuilder::with_vsync is a no-op; present mode hardcoded to Fifo #101

Changes

• Respect window vsync preference when building a RenderContext (when no explicit present mode is set).
• Prefer AutoNoVsync over Mailbox when the caller requests non-vsync presentation but the adapter does not support Immediate.
• Add unit tests covering present mode resolution and platform present-mode fallback ordering.
• Clarify WindowBuilder::with_vsync docs (best-effort based on platform capabilities).

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• Feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (updates to docs, specs, tutorials, or comments)
• Refactor (code change that neither fixes a bug nor adds a feature)
• Performance (change that improves performance)
• Test (adding or updating tests)
• Build/CI (changes to build process or CI configuration)

Affected Crates

• lambda-rs
• lambda-rs-platform
• lambda-rs-args
• lambda-rs-logging
• Other:

Checklist

• Code follows the repository style guidelines (cargo +nightly fmt --all)
• Code passes clippy (cargo clippy --workspace --all-targets -- -D warnings)
• Tests pass (cargo test --workspace)
• New code includes appropriate documentation
• Public API changes are documented
• Breaking changes are noted in this PR description

Testing

Commands run:

cargo test -p lambda-rs-platform --lib
cargo test -p lambda-rs --lib

Manual verification steps (if applicable):

1. (Optional) Run a demo with WindowBuilder::with_vsync(false) and confirm frame pacing is no longer refresh-capped on platforms that support non-vsync modes.

Screenshots/Recordings

N/A

Platform Testing

• macOS
• Windows
• Linux

Additional Notes

[github-actions]

✅ Coverage Report

📊 View Full HTML Report (download artifact)

Overall Coverage

Metric	Value
Total Line Coverage	45.35%
Lines Covered	5064 / 11166

Changed Files in This PR

File	Coverage	Lines
crates/lambda-rs-platform/src/wgpu/surface.rs	26.71%	74/277
crates/lambda-rs/src/render/mod.rs	8.42%	58/689
crates/lambda-rs/src/render/window.rs	0.00%	0/63

PR Files Coverage: 12.82% (132/1029 lines)

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_{Last updated: 2026-02-07 22:01:48 UTC · Commit: 153dc64}