fix: don't duplicate prefixed rule when selector list was reformatted#1552
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ai merged 1 commit intoJul 16, 2026
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The `already()` check compared `before.selector` to the generated
prefixed selector with strict equality. Generated prefixed selectors
always join the comma list with `', '`, so once an existing prefixed
rule had its selector list reformatted (e.g. one selector per line by a
formatter), the comparison failed and Autoprefixer inserted a duplicate
prefixed rule on every run.
Normalize the compared selector with `list.comma(...).join(', ')` so
whitespace differences in the selector list no longer defeat the
duplication check.
Closes postcss#1497
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Thanks. Released in 10.5.4. |
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Summary
Autoprefixer's output was not idempotent for prefixed rules whose selector list had been reformatted: running it again inserted a duplicate prefixed rule on every pass (#1497).
Root cause
Selector.already()decides whether a prefixed rule already exists by comparingbefore.selectorto the generated prefixed selector with strict equality:Generated prefixed selectors always join the comma list with
', '. Once an existing prefixed rule had its selector list reformatted (for example, one selector per line by a code formatter),before.selectorno longer matched the normalized generated string, the duplication check failed, and a duplicate prefixed rule was added.Fix
Normalize the compared selector with
list.comma(before.selector).join(', ')before the comparison, so purely cosmetic whitespace differences in the selector list no longer defeat the duplication check.Tests
Added a test in
test/selector.test.jscoveringalready()when the existing prefixed rule's selector list was reformatted (one selector per line).Fixes #1497