Module cache 

Handling Viewport Resizing and Layout Thrashing

The viewport size is a fundamental input to the entire layout process. A change in viewport size must trigger a relayout.

1. The layout_document function takes the viewport as an argument. The LayoutCache stores the viewport from the previous frame.
2. The reconcile_and_invalidate function detects that the viewport has changed size
3. This single change—marking the root as a layout root—forces a full top-down pass (calculate_layout_for_subtree starting from the root). This correctly recalculates all(calculate_layout_for_subtree starting from the root). This correctly recalculates all percentage-based sizes and repositions all elements according to the new viewport dimensions.
4. The intrinsic size calculation (bottom-up) can often be skipped, as it’s independent of the container size, which is a significant optimization.

Structs

LayoutCache

The persistent cache that holds the layout state between frames.

LayoutCacheEntry

Cache entry for full layout (PerformLayout mode).

LayoutCacheMap

External layout cache, parallel to LayoutTree.nodes.

NodeCache

Per-node cache entry with 9 measurement slots + 1 full layout slot.

ReconciliationResult

The result of a reconciliation pass.

SizingCacheEntry

Cache entry for sizing (ComputeSize mode) — stores NO positions.

Enums

AvailableWidthType

Constraint classification for deterministic cache slot selection.

ComputeMode

Determines whether calculate_layout_for_subtree should only compute the node’s size (for parent’s sizing pass) or perform full layout including child positioning.

Functions

calculate_layout_for_subtree

Recursive, top-down pass to calculate used sizes and positions for a given subtree. This is the single, authoritative function for in-flow layout.

collect_children_dom_ids

Collects DOM child IDs from the node hierarchy into a Vec.

compute_counters

Computes CSS counter values for all nodes in the layout tree.

is_simple_flex_stack

Checks if a flex container is simple enough to be treated like a block-stack for repositioning.

reconcile_and_invalidate

Compares the new DOM against the cached tree, creating a new tree and identifying which parts need to be re-laid out.

reconcile_recursive

Recursively traverses the new DOM and old tree, building a new tree and marking dirty nodes.

reposition_block_flow_siblings

Repositions clean children within a simple block-flow layout (like a BFC or a table-row-group). It stacks children along the main axis, preserving their previously calculated cross-axis alignment.

reposition_clean_subtrees

After dirty subtrees are laid out, this repositions their clean siblings without recalculating their internal layout. This is a critical optimization.

shift_subtree_position

Helper to recursively shift the absolute position of a node and all its descendants.

style_text_align_to_fc

Convert StyleTextAlign to fc::TextAlign

to_overflow_behavior

Convert LayoutOverflow to OverflowBehavior CSS Overflow Module Level 3: initial value of overflow is visible.