LibWeb: Cache recursive scrollable overflow measurement - #11138
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info⚙️ Run configurationConfiguration used: Organization UI Review profile: CHILL Plan: Pro Plus Run ID: 📒 Files selected for processing (2)
📝 WalkthroughWalkthroughThe change adds fragment identities and propagates them through layout commits to invalidate cached overflow data. Paintables store cached padding-relative overflow and expose overflow queries. Document separates full-layout and subtree commits, performs eager overflow measurement, clears stale caches, and reclamps scroll offsets. Scrollable-overflow measurement reuses valid cached data and avoids unnecessary transforms. New tests cover cache validity, resizing, writing directions, dynamic mutations, and varied layout scenarios. Sequence Diagram(s)sequenceDiagram
participant LayoutCommit
participant Document
participant Paintable
participant ScrollableOverflow
LayoutCommit->>Document: Commit full or subtree layout
Document->>Paintable: Clear invalid cached overflow
Document->>ScrollableOverflow: Measure required overflow boxes
ScrollableOverflow->>Paintable: Store measured overflow data
Document->>Paintable: Clamp affected scroll offsets
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Suggested reviewers: Merge Risk: ⚪ Minimal · up to This change caches scrollable overflow measurements to reduce layout cost while covering cache invalidation and complex layout scenarios; no actionable merge-blocking risk remains after normal checks and review. 🚥 Pre-merge checks | ✅ 3✅ Passed checks (3 passed)
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Cover propagation through visible overflow, clipping, containment, positioned and transformed descendants, line fragments, padding, nested scroll containers, replaced SVG viewports, and physical directions. Compare primed overflow caches with cold measurements after indirect size and placement changes. Exercise percentages, constraints, transforms, text wrapping, flex, grid, nested scrollers, writing modes, inherited metrics, aspect ratios, tree rebuilds, containing blocks, and viewport resize. Exercise dynamic style and layout changes, insertion and removal, reused subtree movement, scroll range updates, offset clamping, and cached atomic inlines with padding and borders.
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Inline comments:
In `@Libraries/LibWeb/Painting/Paintable.cpp`:
- Around line 1178-1183: Update Paintable::has_scrollable_overflow() to call
document().ensure_scrollable_overflow_is_measured() before reading
m_overflow_data or m_cached_overflow_data, matching scrollable_overflow_rect()
while preserving the existing flag lookup and return behavior.
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ℹ️ Review info
⚙️ Run configuration
Configuration used: Organization UI
Review profile: CHILL
Plan: Pro Plus
Run ID: 1df23173-ec9e-483e-be0e-8a7cc0f9ac40
📒 Files selected for processing (19)
Libraries/LibWeb/DOM/Document.cppLibraries/LibWeb/DOM/Document.hLibraries/LibWeb/Layout/LayoutRustBridge.cppLibraries/LibWeb/Layout/Node.cppLibraries/LibWeb/Layout/ScrollableOverflow.cppLibraries/LibWeb/Painting/Paintable.cppLibraries/LibWeb/Painting/Paintable.hLibraries/LibWeb/Rust/src/layout/commit.rsLibraries/LibWeb/Rust/src/layout/fragment_tree.rsTests/LibWeb/Text/expected/css/scrollable-overflow-cache-validity.txtTests/LibWeb/Text/expected/css/scrollable-overflow-cache-viewport-resize.txtTests/LibWeb/Text/expected/css/scrollable-overflow-measurement-directions.txtTests/LibWeb/Text/expected/css/scrollable-overflow-measurement-dynamic.txtTests/LibWeb/Text/expected/css/scrollable-overflow-measurement.txtTests/LibWeb/Text/input/css/scrollable-overflow-cache-validity.htmlTests/LibWeb/Text/input/css/scrollable-overflow-cache-viewport-resize.htmlTests/LibWeb/Text/input/css/scrollable-overflow-measurement-directions.htmlTests/LibWeb/Text/input/css/scrollable-overflow-measurement-dynamic.htmlTests/LibWeb/Text/input/css/scrollable-overflow-measurement.html
Cache contributions relative to each padding box, so valid results can survive subtree movement. Invalidate them from layout and overflow dirt, including containing-block ancestors. Associate entries with the immutable layout fragment that produced them. Preserve them when layout replays that fragment. Discard them when an ancestor or viewport relayout commits newly computed geometry. After full layout, start measurement only from the viewport, scroll containers and active scroll offsets. Recursive traversal reuses local caches, while isolated boxes are measured lazily when requested. Skip atomic inline boxes when their cached overflow is covered by the committed line fragment. Avoid stale overflow diffs after full layout, since that layout reset all paintable overflow state. Reduce StyleBench scrollable overflow work from about 943 ms to 85 ms while preserving metrics across clipping and containment barriers.
This was referenced Aug 15, 2026
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Scrollable overflow measurement repeatedly walked the same containing-block subtrees, making the work quadratic.
Cache each box’s overflow contribution relative to its padding box so it can survive subtree movement. Associate cached data with the immutable layout fragment that produced it, preserving results for replayed fragments while invalidating them whenever ancestor or viewport relayout computes new geometry.
After full layout, eagerly measure only the viewport, scroll containers, and boxes with active scroll offsets. Other boxes are measured recursively or lazily when their overflow rectangle is requested.
This reduces StyleBench scrollable overflow work from approximately 943 ms to 85 ms.
Add comprehensive measurement coverage and a cold-versus-primed cache oracle exercising percentage sizing, constraints, transforms, text wrapping, flex and grid layout, nested scrollers, containment, writing modes, aspect ratios, tree rebuilds, containing-block changes, and viewport resizing.