Index style rules by their rightmost simple selector#4163
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computeStyle tested every selector alternative of every rule against every element. RuleIndex buckets each alternative by the id, class, or element type its rightmost compound requires; alternatives without such a key (universal, attribute-only or pseudo-only compounds) go into a remainder bucket consulted for every element. A lookup gathers the buckets the element satisfies and hands the candidates to the matcher in cascade order, so match positions and cascade results are unchanged. RuleIndexTest locks the completeness invariant: over a stylesheet covering every selector shape, the index never drops an alternative the matcher would accept.
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computeStyle tested every selector alternative of every rule against every element, the main cost of a theme swap. A new RuleIndex buckets each alternative by the id, class, or element type its rightmost compound requires (universal, attribute-only and pseudo-only alternatives go into a remainder bucket consulted for every element), and the matcher now runs only on the candidates an element can possibly match, in unchanged cascade order. Styling results are identical: match positions are preserved, and the new RuleIndexTest locks the invariant that the index never drops an alternative the matcher would accept, checked over a stylesheet covering every selector shape.
Measured with a workbench theme-swap benchmark (20 editors, about 4000 widgets, 10 swap rounds per direction), engine applyStyles time fell from 196.6 to 155.6 microseconds per call and from 3.34 to 2.65 seconds per swap, about 21 percent less styling work per theme change, with identical call counts confirming unchanged behavior.
Part of #3980.