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.

ReconciliationResult

The result of a reconciliation pass.

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