Compute_block (p.toffee.0.1.0.doc.compute_block.Compute

Algorithm Overview

Block layout proceeds in phases:

Generate item list from children, resolving styles and filtering display:none nodes
Determine container width from known dimensions or intrinsic content width
Perform final layout on in-flow children with margin collapsing
Layout absolutely positioned children in the containing block
Compute final content size and margin collapse state

Key Invariants

Vertical margins collapse between adjacent in-flow block boxes under specific conditions (no padding/border separation, overflow:visible, position:relative)
Absolutely positioned children are removed from normal flow and sized relative to the containing block
Container width stretches children by default unless they are tables or have explicit width
Auto margins resolve to 0 for vertical margins of in-flow blocks, but expand to fill space for horizontal margins
Margin collapsing can occur through empty blocks with no padding, border, or height

Implementation Details

The algorithm handles margin collapsing via Collapsible_margin_set tracking. In-flow layout maintains two margin sets: first_child_top_margin_set for collapsing with container's top margin, and active_collapsible_margin_set for tracking bottom margins. Nodes can be "collapsed through" when they have no content, padding, border, or height constraints.

Absolutely positioned children use static position as fallback when inset properties are auto. Static position is computed during in-flow layout pass even though the child is not placed in normal flow.

Scrollbar gutters are reserved in the opposite axis (vertical scroll reserves horizontal space). Tables receive special treatment and do not participate in stretch-sizing.

Source

val compute_block_layout : 
  (module Tree.LAYOUT_PARTIAL_TREE with type t = 't) ->
  't ->
  Tree.Node_id.t ->
  Tree.Layout_input.t ->
  Tree.Layout_output.t

compute_block_layout tree_module tree node_id inputs computes block layout for node_id.

Implements CSS block layout with margin collapsing, auto-sizing, and absolute positioning. Returns Layout_output containing final size, content size, and margin collapse state.

Algorithm:

Resolve size constraints from style properties and parent size
Short-circuit if size fully determined and mode is Compute_size
Generate block items from children, filtering display:none
Determine container width (known dimension or intrinsic content-based)
Layout in-flow children with margin collapsing and auto-margins
Layout absolutely positioned children in containing block area
Compute whether node can be collapsed through
Return final size, content size, and margin sets

The implementation tracks margin collapsing state through Collapsible_margin_set. Vertical margins of adjacent in-flow blocks collapse when separated only by collapsible space. Auto horizontal margins expand to fill available space. Absolute children use static position when inset is auto.