Add positionAreaContainingBlock option#428
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Add a `positionAreaContainingBlock` polyfill option that controls whether a `position-area` target is wrapped with an element approximating the containing block that `position-area` natively creates. - `true` (default): always wrap; matches the current behavior most closely. - `false`: never wrap; the polyfill computes and applies inset values directly on the target instead. - `'auto'`: wrap only targets whose styles resolve against the containing block (percentage sizes, `auto`/percentage margins, percentage padding, or `stretch`/`anchor-center` self-alignment); other targets are positioned directly. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Summary
Adds a
positionAreaContainingBlockpolyfill option that controls whether aposition-areatarget is wrapped with an element approximating the containing block thatposition-areanatively creates. The wrapper is correct but can interfere with author CSS that depends on the target's position in the DOM tree (direct-child/sibling combinators, the original parent's flex/grid layout, etc.). This option makes that tradeoff configurable.true(default): always wrap the target — matches the current behavior most closely.false: never wrap; the polyfill computes and applies inset values directly on the target instead. Avoids the extra element, but styles that resolve against the containing block (percentage sizes,auto/percentage margins, percentage padding, orstretch/anchor-centerself-alignment) won't match native behavior.'auto': wrap only targets whose styles resolve against the containing block (same heuristics as above); position all other targets directly.Implementation notes
polyfill.ts: new public option onAnchorPositioningPolyfillOptions(+ normalized). InapplyAnchorPositions, aposition-areatarget positions viawrapper ?? target; unwrapped targets resolve alignment to physical inset values via the newresolveAxisInsetValueshelper.position-area.ts: newPositionAreaContainingBlocktype;isContainingBlockDependentDeclarationheuristic + property lists;addPositionAreaDeclarationBlockStylesemits the wrapped/unwrapped/both declaration sets; newmarkPositionAreaTarget,activeTargetStyles, andPOSITION_AREA_TARGET_ATTRIBUTEfor the unwrapped path. Wrapper insets renamed--pa-value-*→--pa-wrapper-*so a shared'auto'selector's target insets don't collide.parse.ts: collects containing-block-dependent selectors during the AST walk and computesneedsWrapperper target in'auto'mode.Testing
tsc --noEmit: cleaneslint: cleanmarkPositionAreaTarget,dataForPositionAreaTarget,activeTargetStyles)npm run build: succeedsfalse/'auto'rendering was not run and should be confirmed before merge. Porting the e2e tests for the new modes is a follow-up.🤖 Generated with Claude Code