Block main thread when creating a new window

Editor assessment

The type deserves scrutiny: 'block main thread when creating a new window' is an observable behavior change, and refactor is defined as preserving behavior — if this was done to fix a race or flicker, fix or perf would classify it more honestly, which matters for version-bump inference and release-note framing. The subject itself is clear and appropriately short, and the window scope is apt. What is missing is any rationale: blocking the main thread is a contentious choice that screams for a body explaining the trade-off (macOS threading requirements? avoiding unsafe window-handle reuse?). As a record of intent this falls short; as a header, it is competent.