LibWeb: Eliminate some redundant work during repeated layout - #11146
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Exercise 260 combinations across committed subtree movement, escaped absolute and fixed descendants, formatting-context types, nesting depth, inline structural damage, intrinsic sizing, empty atomic roots, viewport units, and iframe viewport changes. Compare geometry with explicit formulas or freshly constructed controls. Report viewport cache reuse as expected output so this coverage can land before the corresponding optimization and make its effect visible. Keep the small reusable matrix so future layout optimizations can extend the same axes.
Avoid constructing and running a block formatting context for a normal atomic inline layout root with no children. Keep root sizing, block-size finalization, and fragment construction on their shared paths. Bypass the run cache too. Building and snapshotting one root directly is cheaper than replaying its cached output.
Cache hits reused formatting-context results but still reset and rebuilt all descendant paintables during commit. This left a full-document commit floor even when most layout runs were cached. Carry reused formatting-context roots through the Rust fragment tree and splice their existing paintable subtrees back into the rebuilt parent. Refresh parent-resolved root metrics and translate absolute geometry and overflow caches when a subtree moves. A later fresh run in the same pass clears an earlier reuse marker. Since commit stops at a reused root, replay only the root outcome and outward-propagated payloads instead of cloning cached descendants. This also avoids a redundant size-query scan caused by resetting an unchanged neighboring query container.
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📝 WalkthroughWalkthroughThis change extends incremental layout reuse across the Rust layout engine and C++ paintable bridge. The run cache tracks structural inline damage without viewport fields. Formatting contexts reuse compatible line data and cached intrinsic sizing results. Commit handling identifies reused subtrees, skips redundant descendant updates, preserves reused paintables, and translates their absolute geometry. Node topology changes propagate inline damage through the layout arena. New tests cover cache reuse, viewport resizing, inline mutations, absolute positioning, intrinsic sizing, and empty atomic elements. Sequence Diagram(s)sequenceDiagram
participant DOMTopology
participant NodeArena
participant RunCache
participant FormattingContext
participant LayoutRustBridge
participant Paintable
DOMTopology->>NodeArena: record inline-layout damage
NodeArena->>RunCache: consume damage during cache probe
RunCache->>FormattingContext: provide cached lines or reused subtree
FormattingContext->>LayoutRustBridge: commit reuse state
LayoutRustBridge->>Paintable: preserve and translate reused geometry
Possibly related PRs
Merge Risk: 🟡 Moderate · up to The change reuses cached inline layout across child-list mutations, but same-parent reordering can preserve stale line ordering or geometry and produce incorrect rendering. This requires owner follow-up or explicit acceptance before merge. 🚥 Pre-merge checks | ✅ 3✅ Passed checks (3 passed)
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Inline comments:
In `@Libraries/LibWeb/Layout/Node.cpp`:
- Around line 158-163: Update the topology-change check in the shown node update
logic to also compare previous_sibling and next_sibling, not only
old_parent.index and new_parent.index. When any parent or sibling slot changes,
notify inline-layout damage for each valid old and new parent, including
same-parent reorders, and add the corresponding inline-damage matrix case.
In `@Libraries/LibWeb/Rust/src/layout/line_builder.rs`:
- Around line 95-107: In LineBuilder::new_after_reused_lines, move the
assignment setting current_line_committed_pending_margin to true until after
begin_new_line returns, so the reused-line initialization preserves the
committed-margin state for the first prepare_to_append_inline_content call.
In `@Libraries/LibWeb/Rust/src/layout/sizing_context.rs`:
- Around line 1157-1187: Update paired_min_content_inline_size_for_atomic_root
and its inline formatting contribution so nowrap contexts use the atomic
element’s max-content inline_size instead of min_content_inline_size; preserve
min-content sizing when wrapping is enabled.
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📒 Files selected for processing (34)
Libraries/LibWeb/Layout/LayoutRustBridge.cppLibraries/LibWeb/Layout/LayoutRustBridge.hLibraries/LibWeb/Layout/Node.cppLibraries/LibWeb/Layout/NodeArena.cppLibraries/LibWeb/Layout/NodeArena.hLibraries/LibWeb/Painting/Paintable.cppLibraries/LibWeb/Painting/Paintable.hLibraries/LibWeb/Rust/src/layout/abspos_engine.rsLibraries/LibWeb/Rust/src/layout/block_formatting_context.rsLibraries/LibWeb/Rust/src/layout/commit.rsLibraries/LibWeb/Rust/src/layout/fc_run_cache.rsLibraries/LibWeb/Rust/src/layout/flex_formatting_context.rsLibraries/LibWeb/Rust/src/layout/formatting_context.rsLibraries/LibWeb/Rust/src/layout/fragment_tree.rsLibraries/LibWeb/Rust/src/layout/grid_formatting_context.rsLibraries/LibWeb/Rust/src/layout/inline_formatting_context.rsLibraries/LibWeb/Rust/src/layout/inline_level_iterator.rsLibraries/LibWeb/Rust/src/layout/layout_node_arena.rsLibraries/LibWeb/Rust/src/layout/line_builder.rsLibraries/LibWeb/Rust/src/layout/sizing_context.rsLibraries/LibWeb/Rust/src/layout/svg_formatting_context.rsLibraries/LibWeb/Rust/src/layout/table_formatting_context.rsTests/LibWeb/Text/expected/css/style-invalidation/size-query-container-scans.txtTests/LibWeb/Text/expected/layout-run-cache-escaped-abspos.txtTests/LibWeb/Text/expected/layout-run-cache-inline-damage.txtTests/LibWeb/Text/expected/layout-run-cache-optimization-scenarios.txtTests/LibWeb/Text/expected/layout-run-cache-viewport-resize-scenarios.txtTests/LibWeb/Text/expected/layout-run-cache-viewport-resize.txtTests/LibWeb/Text/input/layout-optimization-test-matrix.jsTests/LibWeb/Text/input/layout-run-cache-escaped-abspos.htmlTests/LibWeb/Text/input/layout-run-cache-inline-damage.htmlTests/LibWeb/Text/input/layout-run-cache-optimization-scenarios.htmlTests/LibWeb/Text/input/layout-run-cache-viewport-resize-scenarios.htmlTests/LibWeb/Text/input/layout-run-cache-viewport-resize.html
Keep previous line data for formatting-context runs invalidated only by child-list edits. Propagate structural damage accounting from the direct parent through the same ancestor chain as fragment cache epochs. Match complete atomic-inline lines by live node identity and geometry, then restart line construction at the first damaged line. Preserve parent-margin state when resuming after reused lines. Retain the exact epoch-delta check so concurrent style or layout invalidations safely fall back to full line construction. Cover insertion, removal, and same-parent reordering across inline wrapping boundaries.
Let viewport-dependent computed style invalidation and changed layout inputs invalidate formatting-context runs instead of stamping every cache entry with the document viewport. Preserve fixed-size descendant runs during resize while continuing to invalidate viewport-unit and media-query-dependent layout. Add coverage for both paths inside a resized child viewport.
Reuse max-content inline itemization to derive a min-content companion for eligible horizontal flows. Cache the companion with the existing measurement so later min-content queries avoid formatting descendants again. Keep exact min-content cache entries authoritative. Fall back for formatting features whose min-content inputs differ, including box-model metrics, tabs, preserved whitespace, scroll containers, and vertical writing modes. Cover wrapping and nowrap flows with direct, nested inline, and nested atomic content, including accumulation beside a shrinkable atomic item.
This was referenced Aug 16, 2026
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This produces a 1.2x speedup on StyleBench by eliminating work that survived formatting-context cache hits and by reusing more results across closely related layout passes.
Subtree reuse falls back to complete fragment replay when out-of-flow descendants escape to an external containing block, ensuring freshly positioned geometry is committed.
A reusable 260-case scenario matrix covers formatting-context types, nesting depths, subtree movement, absolute and fixed descendants, inline structural damage, intrinsic sizing, empty atomic roots, viewport units, and iframe resizing.