[shared] rework compound pseudos priority logic

[mellyeliu]

Context

This is an interim fix for #1473 to unblock some internal experiments. The larger bug requires a better solution at the API design level.

We currently enforce outer-key pseudo-elements, which by our sorting logic outputs

.x1qdmp1q::before:hover{color:red}

In the meantime, let's allow for compound selectors like :hover::after and sum up the priorities. We'll opt out of handling functional pseudo-classes (eg. :where(), :is(), :has()), as those don't follow additive specificities.

This PR is a follow up to ca12618 to clean up failing signals and refine logic

[vercel]

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Project	Deployment	Actions	Updated (UTC)
stylex	Ready	Preview, Comment	Jan 31, 2026 2:35pm

[github-actions]

workflow: benchmarks/perf

Comparison of performance test results, measured in operations per second. Larger is better.
yarn workspace v1.22.22
yarn run v1.22.22
$ node ./compare.js /tmp/tmp.T9ZOHzST1T /tmp/tmp.0gZa15adOB

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	543	536	0.99	-
· complex create	68	67	0.99	-
babel-plugin: stylex.createTheme
· basic themes	442	435	0.98	-
· complex themes	35	33	0.94	!!
Done in 0.07s.
Done in 0.31s.

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.
yarn workspace v1.22.22
yarn run v1.22.22
$ node ./compare.js /tmp/tmp.s8QvIQENB6 /tmp/tmp.qazKJrT2R0

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,311	1,311	1.00
· minified	4,150	4,150	1.00
@stylexjs/stylex/lib/cjs/inject.js
· compressed	1,793	1,793	1.00
· minified	4,915	4,915	1.00
benchmarks/size/.build/bundle.js
· compressed	496,650	496,650	1.00
· minified	4,847,840	4,847,840	1.00
benchmarks/size/.build/stylex.css
· compressed	99,867	99,867	1.00
· minified	747,613	747,613	1.00
Done in 0.07s.
Done in 0.32s.