Anonymous table box construction fixups (CSS 2.2 §17.2.1)

[nicoburns]

Summary

Follow-up to #781 (based on its branch) implementing the anonymous table box fixups of CSS 2.2 §17.2.1 with real anonymous nodes instead of #781's one-pseudo-cell-per-child simplification.

Generalized anonymous box creation (construct.rs): create_anonymous_block is now a thin wrapper over

create_anonymous_node(doc, container, kind: AnonKind /* Block | Table | TableRow | TableCell */)

which styles the node via stylo's precomputed ServoAnonymousBox/Table/TableRow/TableCell pseudo-elements. Since precomputed pseudos take their declarations from UA rules, default.css gains ::-servo-anonymous-table { display: table } (etc.), so the anonymous boxes carry honest computed displays.

Fixups inside tables (table.rs): collect_table_cells is rewritten as a TableBuilder walker with open_anon_row / open_anon_cell run state:

• a consecutive run of non-table-internal children (blocks, flex/grid, nested tables, non-whitespace text) shares a single anonymous cell (real node, so it gets paint/hit-test geometry);
• cells — real or generated — occurring outside a row share a single anonymous row;
• runs are closed by any table-internal sibling (row, row group, real cell); display: contents stays transparent to runs.

build_table_context returns the generated node ids, which are registered in LayoutChildren::anonymous_blocks so the existing reconstruction/deallocation lifecycle applies unchanged. Real-cell-only behavior (colspan/rowspan attrs, first_cell_border) is preserved.

Missing-parent fixups (construct.rs): flow/flex/grid child collection now wraps consecutive runs of table-internal children (display: table-cell/table-row/row-group/...) in an anonymous display: table box (LayoutChildren::anonymous_table_id), which then flows through normal table construction. Whitespace between table-internal boxes is discarded rather than closing the run, per spec.

Testing

New tests/blitz-tests/tests/table_box_fixups.rs covers: run-sharing of misplaced children, real cells closing runs, anonymous rows around bare cells, bare text under tables, stray cells/rows in block containers, incremental mutation geometry updates, and anonymous-node leak checks across 20 reconstructions. cargo fmt / clippy --workspace / test --workspace all pass.

Regression triage (vs main baseline)

Fixed in this PR (follow-up commit):

• Captions (s-11-1-1b-001/008/009, caption-margin-002/005, contain-size-056, contain-size-table-caption-001, content-visibility-094/095, table-as-item-flex-cross-size, table-as-item-stretch-cross-size{,-2}): TableBuilder was pushing display: table-caption children into anonymous cells; they are now excluded from the grid (matching pre-PR behavior — full caption layout needs the table wrapper box and remains future work).
• Replaced elements (outline-applies-to-016/017): CSS 2.2 table box generation applies to non-replaced elements only, so internal-table display on e.g. <img> no longer triggers the missing-parent anonymous-table wrapping.
• Also hardened invalidation: changes to border-spacing/border-collapse etc. now mark box-tree rebuild damage, since these are baked into TableContext at construction time.

Not regressions (fail identically on main in a local run; likely CI environment/flakiness): css/WOFF2/*.

Remaining, out of scope for this PR:

• bidi-008b, inline-table-002a: previously the misplaced table content wasn't rendered at all, so test and ref trivially matched; the anon boxes now render it and expose pre-existing bidi/letter-spacing and inline-table baseline issues.
• table-anonymous-border-spacing: requires script support (mutates style.borderSpacing from JS); the static rendering is now more correct.
• transform-transformable-table-{row,row-group,header-group,footer-group}: transforms on stray table-internal boxes wrapped in an anonymous table; needs transform support on internal table boxes.
• break-before-second-row: fragmentation.

WPT results

61 newly passing, 8 newly failing (net +53).

Full diff (69 changed tests)

- Pass => Fail css/CSS2/bidi-text/bidi-008b.xht
+ Fail => Pass css/CSS2/bidi-text/bidi-009a.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-001.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-001a.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-002.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-003.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-004.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-004a.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-005.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-006.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-007.xht
+ Fail => Pass css/CSS2/floats-clear/float-applies-to-015.xht
- Pass => Fail css/CSS2/normal-flow/inline-table-002a.xht
+ Fail => Pass css/CSS2/normal-flow/inline-table-width-001a.xht
+ Fail => Pass css/CSS2/normal-flow/inline-table-width-001b.xht
+ Fail => Pass css/CSS2/normal-flow/inline-table-width-002a.xht
+ Fail => Pass css/CSS2/normal-flow/inline-table-zorder-004.xht
+ Fail => Pass css/CSS2/normal-flow/inline-table-zorder-005.xht
+ Fail => Pass css/CSS2/tables/caption-side-applies-to-017.xht
- Pass => Fail css/CSS2/tables/table-anonymous-border-spacing.xht
+ Fail => Pass css/CSS2/tables/table-margin-004.xht
- Pass => Fail css/css-break/table/break-before-second-row.html
+ Fail => Pass css/css-break/table/monolithic-overflow-002.tentative.html
+ Fail => Pass css/css-break/table/repeated-section/abspos-uncontained-text.html
+ Fail => Pass css/css-break/table/repeated-section/background-001.tentative.html
+ Fail => Pass css/css-break/table/repeated-section/image.tentative.html
+ Fail => Pass css/css-break/table/table-border-002.html
+ Fail => Pass css/css-break/table/table-border-005.html
+ Fail => Pass css/css-contain/contain-inline-size-table.html
+ Fail => Pass css/css-contain/contain-size-012.html
+ Fail => Pass css/css-contain/contain-size-012b.html
+ Fail => Pass css/css-display/run-in/run-in-inline-table-between-001.xht
+ Fail => Pass css/css-display/run-in/run-in-table-between-001.xht
+ Fail => Pass css/css-flexbox/flexbox-align-self-horiz-001-table.xhtml
+ Fail => Pass css/css-flexbox/flexbox_flex-formatting-interop.html
+ Fail => Pass css/css-flexbox/flexbox_stf-table-cell.html
+ Fail => Pass css/css-flexbox/flexbox_stf-table-row-group.html
+ Fail => Pass css/css-flexbox/flexbox_stf-table-row.html
+ Fail => Pass css/css-flexbox/flexbox_stf-table-singleline-2.html
+ Fail => Pass css/css-flexbox/flexbox_stf-table-singleline.html
+ Fail => Pass css/css-flexbox/table-as-item-narrow-content.html
+ Fail => Pass css/css-flexbox/table-item-flex-percentage-min-width.html
+ Fail => Pass css/css-flexbox/table-item-flex-percentage-width.html
+ Fail => Pass css/css-flexbox/table-with-float-paint.html
+ Fail => Pass css/css-grid/grid-items/explicitly-sized-grid-item-as-table.html
+ Fail => Pass css/css-page/monolithic-overflow-009-print.html
+ Fail => Pass css/css-page/monolithic-overflow-010-print.html
+ Fail => Pass css/css-page/monolithic-overflow-011-print.html
+ Fail => Pass css/css-sizing/table-child-percentage-height-with-border-box.html
+ Fail => Pass css/css-sizing/table-percentage-max-width-beside-float.html
+ Fail => Pass css/css-tables/anonymous-table-cell-margin-collapsing.html
+ Fail => Pass css/css-tables/anonymous-table-ws-001.html
+ Fail => Pass css/css-tables/dynamic-table-cell-height.html
+ Fail => Pass css/css-tables/html-display-table.html
+ Fail => Pass css/css-tables/percent-height-overflow-auto-in-unrestricted-block-size-cell.tentative.html
+ Fail => Pass css/css-tables/percentages-grandchildren-quirks-mode-001.html
+ Fail => Pass css/css-tables/percentages-grandchildren-quirks-mode-002.html
+ Fail => Pass css/css-tables/table-row-group-color-inheritance-001.html
+ Fail => Pass css/css-tables/whitespace-001.html
+ Fail => Pass css/css-transforms/transform-abspos-006.html
+ Fail => Pass css/css-transforms/transform-abspos-007.html
+ Fail => Pass css/css-transforms/transform-display-002.html
+ Fail => Pass css/css-transforms/transform-display-003.html
- Pass => Fail css/css-transforms/transform-transformable-table-footer-group.html
- Pass => Fail css/css-transforms/transform-transformable-table-header-group.html
- Pass => Fail css/css-transforms/transform-transformable-table-row-group.html
- Pass => Fail css/css-transforms/transform-transformable-table-row.html
+ Fail => Pass css/css-ui/outline-011.html
+ Fail => Pass css/css-ui/outline-019.html

_{Generated by the WPT workflow.}

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