For changes happening in stylesheets, the header of the source in the Changes panel should show only the stylesheet filename instead of the full URL. This improves readability. Stylesheets from iframes should be identified as applied on the iframe instead of the parent document.

Depends on D12588 Support indefinite nesting by declaring the --diff-level CSS custom property as an inline style on the element at its level of nesting. This custom property will be used in the stylesheet to compute the appropriate padding-left to create the indentation necessary. A minumum offset is used, --diff-level-min-offset, to account for the first level (zero-indexed) and to allow additional padding on added/removed lines to clear the +/- icons.

Depends on D12590

Pushed by rcaliman@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/e2d3dd273e41 Trim source URLs to filename in Changes panel. r=gl https://hg.mozilla.org/integration/autoland/rev/999546476a96 Move level definition to CSS variable in DOM and reuse in Changes panel stylesheet. r=gl https://hg.mozilla.org/integration/autoland/rev/677d32e4a6af Mark style sources from iframes in the Changes panel. r=gl

https://hg.mozilla.org/mozilla-central/rev/e2d3dd273e41 https://hg.mozilla.org/mozilla-central/rev/999546476a96 https://hg.mozilla.org/mozilla-central/rev/677d32e4a6af