[Gherkin]

Fixes #131. Fixes #130.

The buffer is see-through outside the board outline. The backdrop under it showed there all the time once it had been drawn:

• The blurry backdrop shows through outside the board outline, even when the sharp buffer covers the view #131: at a high zoom, as a blurred copy of the edge line.
• Old backdrop shows at the board edge after a layer or colour toggle #130: after a layer or colour toggle, in the old state, until the new backdrop came.

The backdrop is now displayed only when the drawn part of the buffer does not cover the whole view. It is the same transform updateCSSTransform() uses.

• bufferCoversView() in render.js. While overscan pieces are still to come, only the viewport rect of the render counts as drawn. When all pieces are in, the whole buffer counts.
• updateBackdropTransform() sets display from that. It already runs on every pan, zoom and blit. blitPiece() calls it when the last piece is in.
• blitBackdrop() sets _backdropReady instead of display: block. hideBackdrop() clears it.

This differs from the fix in #131, which waits for all overscan pieces before it hides the backdrop. Counting the viewport rect hides it at the first blit after a toggle, and that is what fixes #130. Hiding the backdrop on every key change, as #130 suggests, was not needed.

Tests

• New: the backdrop is not displayed while the buffer covers the view, is displayed after a fast zoom out, and is hidden again after the renders.
• New: with the worker's backdrop held back, a press on S leaves the old backdrop hidden.
• The tests that used display === 'block' to mean "a backdrop for this view is drawn" now check _backdropReady. The resize test records hideBackdrop() calls, since display may already be none.
• Both new tests fail on main.
• Full Playwright suite: 132 passed, 1 timed out (an inner layer checkbox changes both sides). That test passes when run alone; it is the known flaky one. npm run test:api passes.

🤖 Generated with Claude Code

https://claude.ai/code/session_01UCbPYsA5LYNX8HZaGGZCpK

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-10-27 15:55 UTC

[Gherkin]

Review, round 1.

• web/render.js:319-321 bufferCoversView() divides by devicePixelRatio and compares to clientWidth. At a pixel ratio of 1.1 and a 1000 px wide view, the viewport rect gives 999.9999999999999 < 1000, so the view never counts as covered while pieces are pending, and the old backdrop shows after a toggle (Old backdrop shows at the board edge after a layer or colour toggle #130 at that ratio). Fix: compare in device pixels against clientWidth * devicePixelRatio, the same width postRender() sends the worker.