css: parse ::view-transition-group-children() as a view transition pseudo-element - #42779
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…eudo-element Add the PseudoElement::ViewTransitionGroupChildren variant from css-view-transitions-2. The argument uses ViewTransitionPartName::parse, like the other four view transition pseudo-elements. In a CSS module the name and the class get the module hash, and the class is exported. :only-child is accepted after it. Fixes #42777
WalkthroughThe CSS selector parser now supports ChangesView-transition group children
Suggested reviewers: Priority: ⬇️ Low Merge Risk: 🟡 Moderate · up to CSS Modules can emit 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@src/css/selectors/parser.rs`:
- Around line 1273-1274: Update the ViewTransitionGroupChildren parsing branch
to use a name-only parser that accepts only * or <custom-ident>, rather than
ViewTransitionPartName::parse and its class-selector support. Adjust the related
tests to remove class hashing and export assertions for this variant while
preserving class support for the other view-transition pseudo-elements.
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📒 Files selected for processing (4)
src/css/selectors/parser.rssrc/css/selectors/selector.rstest/bundler/css/css-modules.test.tstest/bundler/css/view-transition-23600.test.ts
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LGTM — mechanical addition of a fifth view-transition pseudo-element variant, following the existing four exactly.
What was reviewed:
- Audited every
match/matches!onPseudoElementfor missed arms — the new variant is wired into the parser length-gate, serializer,Display, andis_view_transition(); remaining sites use_ =>catch-alls or derive-generated eql/hash/clone. - Verified the 30-byte length gate matches
"view-transition-group-children"and the PERF comment count is consistent. - Tests cover name/class/wildcard arguments,
:only-childsuffix, CSS-module hashing + exports, and the no-warning contract; they usetempDir/itBundledper harness conventions and the changed paths have no CODEOWNER.
Extended reasoning...
Overview
This PR adds ::view-transition-group-children() (CSS View Transitions Level 2) as a recognized functional pseudo-element in Bun's CSS parser. It introduces one new PseudoElement enum variant carrying a ViewTransitionPartName, and threads it through the four sites that already enumerate the sibling ViewTransitionGroup/ImagePair/Old/New variants: the length-gated functional-pseudo-element parser (src/css/selectors/parser.rs), the is_view_transition() predicate, the fmt::Display impl, and the serializer arm in src/css/selectors/selector.rs. Two tests are added — one itBundled CSS-modules case and one Bun.build round-trip case.
Security risks
None. This is a CSS selector recognizer that reuses the existing ViewTransitionPartName::parse argument parser verbatim; no new untrusted-input arithmetic, allocation sizing, or FFI surface is introduced. The serializer writes a fixed literal plus the already-escaped part-name via the shared to_css path.
Level of scrutiny
Low-to-moderate. The change is small, self-contained, and purely additive: a fifth entry in a set of parallel switch arms that already handle four structurally identical siblings. Per REVIEW.md's "one source of truth" rule I grepped for every match on PseudoElement and every reference to the sibling ViewTransition* variants across src/ — all enumerating sites are updated, and the remaining match sites (get_prefix, get_necessary_prefixes, valid_after_slotted, is_unknown, is_webkit_scrollbar) correctly fall through _ for this variant. CssEql/CssHash are derived and deep_clone delegates to Clone, so no manual eql/hash/clone arm is needed; ViewTransitionPartName is already bridged in generics.rs.
Other factors
The 30-byte length gate is correct for the literal, and the PERF comment's entry list was updated in step. Tests follow repo conventions (tempDir from harness, itBundled, no network, no sleeps) and cover the full argument variant matrix (custom-ident, .class, *) plus the :only-child suffix that is_view_transition() enables, along with the CSS-module hashing/export behavior that motivated the fix. CODEOWNERS covers only *.d.ts, so none of the changed files require owner sign-off. Bug-hunt exit reason was dry_streak with no findings and no ruled-out candidates.
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Updated 1:07 AM PT - Sep 15th, 2026
✅ @robobun, your commit ba63d8c8f2ee5b1810aa2d4fac8265b0ed8de2dd passed in 🧪 To try this PR locally: bunx bun-pr 42779That installs a local version of the PR into your bun-42779 --bun |
…selector Since #42779, `::view-transition-group-children(hero.big)` stops the build with `error: Unexpected token: .`. The pseudo-element now takes the same `ViewTransitionPartSelector` as the other four view transition pseudo-elements: a name or `*` followed by classes, or classes alone.
2 participants
Problem
::view-transition-group-children()(css-view-transitions-2) is not a known pseudo-element.bun buildwarnsInvalid selector. Unsupported pseudo-class or pseudo-element 'view-transition-group-children'on valid CSS.PseudoElement::CustomFunctioninparse_functional_pseudo_element(src/css/selectors/parser.rs:1216). That arm keeps the raw tokens. In a CSS module the argument misses the module hash, so::view-transition-group-children(hero)can never matchview-transition-name: hero_<hash>. A class used only in that selector is missing from the exports object.Fix
PseudoElement::ViewTransitionGroupChildren { part_name }variant. The parser arm parses the argument withViewTransitionPartName::parse, the same function the four existing view transition pseudo-elements use. That function hashes the name and the class in a CSS module and records the class export.src/css/selectors/selector.rs, theDisplayarm, and theis_view_transitionarm. The last one lets:only-childfollow the pseudo-element, as in Chromium.test/bundler/css/css-modules.test.ts(css-module/ViewTransitionGroupChildrenScoped) andtest/bundler/css/view-transition-23600.test.ts(the new#42777test). Both fail on the released bun and pass with this change. All oftest/bundler/css/passes (180 tests).Background
parse_functional_pseudo_elementmatches a functional pseudo-element by name length first, then by name. The new arm is the 30-byte entry.ViewTransitionPartNameis the argument of::view-transition-group()and its siblings:*, a custom ident, or.class. In a CSS module,new_local_identifiergives the class the module hash and an export entry (css: export view-transition pseudo-element classes from CSS modules #42729).ViewTransitionPartNamewith a name plus class list for the existing four pseudo-elements. If it lands, the new variant takes the same type in a one-line follow-up.Notes
Repro from the issue with this change:
No warning. The plain CSS test uses
Bun.buildand checksresult.logs, because a warning from a CSS entry point is printed without a source location anditBundleddrops warnings that have no location.The warning-only cases in the issue (
:active-view-transition-type()and@view-transition) produce correct output and are not part of this change.[human-review] gate passed · iteration 0 · 4 files touched
fails on main (without fix)
passes on PR (with fix)
diff hotspot
gate history · 1 passed · 0 rejected · iteration 0
evidence per changed file
root cause · written by the author bot
The CSS selector parser's
parse_functional_pseudo_elementrecognized only four named view transition pseudo-elements, so::view-transition-group-children()fell through toPseudoElement::CustomFunction, which emitted an "unsupported pseudo-element" warning and preserved the raw argument tokens; in a CSS module that meant the name or class argument never received the module hash, leaving the rule unable to match the hashedview-transition-nameand omitting the class from the exports object. The fix adds aViewTransitionGroupChildrenvariant that parses its argument through the shar…