rule_order: canonicalise layer declarations to one normal form - #1275
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Two sheets that establish the same layer order in the same places project to two different canonical strings, because the optimizer folds an empty layer block into a trailing declaration while the pin logic splits a leading multi-name statement. The canonical projection then reports the two render-equal sheets as a difference; Tailwind's preflight, whose layers are declared in one statement, reads apart from tw's block-by-block sheet. The projection is a canonical form, so it must be confluent: one output per class of sheets that render alike. Read each block as runs separated by a barrier, a statement that is not a layer itself but declares a layer below it, such as an @media holding an @layer or an @import with a layer, whose contribution depends on its condition. Within a run, write every layer name the run introduces, each a.b path expanded to a and a.b, once as one `@layer <names>;` statement at the run's first name-bearing construct, drop the run's other declarations and empty blocks, and put its style-only blocks in rank order. The order is unchanged and nothing crosses a barrier. The fuzz suite pins the two properties: respellings of one order converge, and projecting the projection changes nothing.
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The canonical projection was not confluent for layer declarations: Tailwind's preflight, whose layers are declared in one
@layer a, b;statement, read apart from tw's block-by-block sheet though the two render the same. It now writes one@layer <names>;statement per conditional-free run, eacha.bpath expanded toathena.b, and puts the run's style-only blocks in rank order. Nothing is hoisted across a conditional group that declares a layer. A fuzz suite pins convergence and idempotence, and aCss_comparecase pins the preflight shape.