fix: cover an already-loaded image with its block bone (BON-15)

[hunterbecton]

Closes BON-15.

What changed

A loaded opaque image paints over its own background-color, so an <img> with a resolved src inside a busy region (auto.css) or one marked data-bone="block" (bones.css) showed the picture instead of a bone. Replaced elements render no pseudo-elements, which is why the background approach was used in the first place.

Both block rules now set object-position: 9999px 0. With the default object-fit: fill, the picture is pushed out of its content box and clipped, while the box and its bone background keep painting. Verified in Chromium before choosing it; object-fit: none plus a negative offset also works but is one more property for the same result. object-position is interpolable, so the rules also set transition: none, which keeps a page transition: all on images from sliding the picture out on the way into the busy state. On the way out the page's own transition applies and the picture reappears at the end of it rather than the start.

Covered: img, canvas, and video without native controls. Not covered, and documented in both READMEs: iframe, embed, object (they ignore object-position), video controls (they paint over the bone), and page CSS that sets object-position itself, which outranks the layered auto rule the same way color does.

The issue scopes itself to auto.css. bones.css has the same defect for marked bones, so it gets the same fix here rather than a twin issue.

Test

tests/browser/img-occlusion.test.ts. Computed style cannot see paint, so the test screenshots the element with locator.screenshot({ base64: true, save: false }), decodes it on a canvas, and samples the center pixel. The expected value is the element's computed background-color composited over the white page, within 2 per channel, so a regression in the --bone-base derivation fails it too. Three cases:

• Control: a loaded red image outside a busy region samples pure red. This proves the harness sees the image; without it a passing test could mean the screenshot was blank.
• auto.css: the same image inside aria-busy="true" samples the bone color.
• bones.css: the same image with data-bone="block" samples the bone color.

packages/bones/.vitest-attachments/ (failure captures, previously untracked noise) is now gitignored.

vp test --run in packages/bones: 211 passed. The visual baselines use a src-less <img> from the BON-4 workaround, so they should not move; CI's Linux comparison is the check.

Follow-up

BON-25 (filed today) proposes folding auto.css into @camp.dev/bones/css. This fix would have been one change under that layout.

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📝 Walkthrough

Walkthrough

Block bones now cover already-loaded img, video, and canvas content by positioning it outside the content box. Browser tests verify automatic and explicit block bones. Documentation and the changeset describe the behavior.

Changes

Loaded media bone occlusion

Layer / File(s)	Summary
Position loaded media outside block bones packages/bones/src/css/auto.css, packages/bones/src/css/bones.css	The CSS rules add object-position: 9999px 0 for block bones. Loaded img, video, and canvas elements retain the bone background. Loaded iframe, embed, and object content remains visible.
Verify and document media occlusion packages/bones/tests/browser/img-occlusion.test.ts, README.md, packages/bones/README.md, .gitignore, .changeset/block-bones-cover-loaded-images.md	Browser tests compare center pixels for uncovered images and images inside automatic or explicit block bones. Documentation, test artifact ignores, and the patch changeset record the behavior.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to 5faec

The change hides loaded images and videos behind their bones, but marked canvas elements are not covered by the same rule and may still display their content instead of the bone. The PR is otherwise mergeable with explicit owner follow-up to include canvas in the selector.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files. (6 skipped: 6…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main fix: covering already-loaded images with their block bone. The BON-15 reference is relevant.
Description check	✅ Passed	The description explains the problem, implementation, supported and unsupported elements, test method, test results, and related follow-up work. It uses different headings from the template, but it pr…

Full details: Docstring Coverage

Explanation

Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 1 files. (6 skipped: 6 unsupported.)

Full details: Description check

Explanation

The description explains the problem, implementation, supported and unsupported elements, test method, test results, and related follow-up work. It uses different headings from the template, but it provides the required What/Why and Testing information.

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Inline comments:
In `@packages/bones/src/css/bones.css`:
- Line 77: Update the explicit block-bone selector in bones.css to include
canvas alongside img and video, ensuring canvas[data-bone="block"] receives the
same object-position styling and cannot obscure the bone background.

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📥 Commits

Reviewing files that changed from the base of the PR and between d2cebaa and 5faec77.

📒 Files selected for processing (7)

• .changeset/block-bones-cover-loaded-images.md
• .gitignore
• README.md
• packages/bones/README.md
• packages/bones/src/css/auto.css
• packages/bones/src/css/bones.css
• packages/bones/tests/browser/img-occlusion.test.ts

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