[ekoneil]

What changed

Launching stet file.md from a terminal on macOS could open a blank window
that only drew the document after a click. With STET_TIMING=1 stet -f, the
first redraw was requested three times and laid out each time, but no surface
texture was ever acquired and nothing asked again:

stet:  95.61ms  ready for first frame
stet: 100.31ms  first redraw requested
stet: 100.53ms  first frame laid out
stet: 102.37ms  first redraw requested
stet: 102.54ms  first frame laid out
stet: 118.64ms  first redraw requested
stet: 118.69ms  first frame laid out

redraw() returned on Occluded and Timeout without requesting another
frame, and there was no handler for WindowEvent::Occluded, so the dropped
frame was never retried until input arrived.

• WindowEvent::Occluded(false) now requests a redraw.
• Timeout requests another frame instead of dropping it.
• Occluded still returns without retrying, so a hidden or minimized window
  doesn't spin.

Verification

• Manual, macOS (Apple Silicon): before, the window was blank until clicked;
  after, tour.md draws on launch, and the timing log reaches
  first frame presented with no input.
• No automated test: the bug needs a real window and surface plus on-screen
  timing, which --screenshot (headless) doesn't create. Happy to add one if
  --drive can reproduce it.
• cargo fmt --check, cargo clippy --all-targets --all-features -D warnings,
  cargo doc with -D warnings, and cargo test --all-targets --all-features
  (307 passed, 2 ignored) all pass locally.
• cargo audit fails on main too (RUSTSEC-2026-0433, pulldown-cmark 0.13.5);
  unrelated to this change, and addressed in a separate PR.

Checklist

• Tests cover the behavior change (see above: manual only)
• Documentation is updated where needed (no behavior or config change)
• No secrets, private fixtures, or generated artifacts are included
• Formatting, clippy, tests, and docs pass locally

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