Fix relative URL resolution in HTML page reports

[davidarny]

Summary

Fix relative URL resolution in generated HTML page reports by adding a <base> tag for per-page summary/run pages.

This makes page reports work whether they are opened as:

• /pages/example_com/path
• /pages/example_com/path/
• /pages/example_com/path/index.html

Without this, relative URLs like data/browsertime.har.gz and data/screenshots/... can resolve to the wrong parent path when the page is opened without a trailing slash.

Problem

Generated page report HTML uses relative paths such as:

• data/browsertime.har.gz
• data/screenshots/...
• ./1.html
• metrics.html

When a report page is opened at a slashless directory URL, the browser resolves those relative paths against the parent path, which can produce 404s for the HAR and screenshots. The waterfall then stays loading forever because the fetch/decompression never succeeds.

Fix

• Add optional baseHref support in the shared HTML layout template.
• Pass baseHref for URL summary and URL run pages from htmlBuilder.js.

The computed base href points to the generated page directory, so all existing relative URLs continue to work correctly.

Related

Fixes #4608

[chatgpt-codex-connector]

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