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layout: Simplify PositioningContext by having it hold a single Vec (#36795)

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`PositioningContext` held two vectors, one inside an `Option`, to
differentiate between the version used for a containing block for all
descendants (including `position: absolute` and `position: fixed`) or
only for `position: absolute` descendants. This distinction was really
hard to reason about and required a lot of bookkeeping about what kind
of `PositioningContext` a layout box's parent expected. In addition, it
led to a lot of mistakes.

This change simplifies things so that `PositioningContext` only holds a
single vector. When it comes time to lay out hoisted absolutely
positioned
fragments, the code then:
 - lays out all of them (in the case of a `PositioningContext` for all
   descendants), or
 - only lays out the `position: absolute` descendants and preserves the
   `position: fixed` descendants (in the case the `PositioningContext`
   is only for `position: absolute`.), or
- lays out none of them if the `PositioningContext` was created for
  box that did not establish a containing block for absolutes.

It's possible that this way of dealing with hoisted absolutes is a bit
less efficient, but, the number of these descendants is typically quite
small, so it should not be significant. In addition, this decreases the
size in memory of all `PositioningContexts` which are created in more
situations as time goes on.

Testing: There is a new WPT test with this change.
Fixes: #36696.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
This commit is contained in:
Martin Robinson 2025-05-02 14:20:11 +02:00 • committed by GitHub
parent e25e63b587
commit 9bc16482a3
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GPG key ID: B5690EEEBB952194
12 changed files with 203 additions and 323 deletions
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47
components/layout/table/layout.rs
View file

@ -1068,7 +1068,6 @@ impl<'a> TableLayout<'a> {
&mut self,
layout_context: &LayoutContext,
containing_block_for_table: &ContainingBlock,
parent_positioning_context: &mut PositioningContext,
) {
self.cells_laid_out = self
.table
@ -1076,30 +1075,6 @@ impl<'a> TableLayout<'a> {
.par_iter()
.enumerate()
.map(|(row_index, row_slots)| {
// When building the PositioningContext for this cell, we want it to have the same
// configuration for whatever PositioningContext the contents are ultimately added to.
let collect_for_nearest_positioned_ancestor = parent_positioning_context
.collects_for_nearest_positioned_ancestor() ||
self.table.rows.get(row_index).is_some_and(|row| {
let row = row.borrow();
let row_group_collects_for_nearest_positioned_ancestor =
row.group_index.is_some_and(|group_index| {
self.table.row_groups[group_index]
.borrow()
.base
.style
.establishes_containing_block_for_absolute_descendants(
FragmentFlags::empty(),
)
});
row_group_collects_for_nearest_positioned_ancestor ||
row.base
.style
.establishes_containing_block_for_absolute_descendants(
FragmentFlags::empty(),
)
});
row_slots
.par_iter()
.enumerate()
@ -1141,10 +1116,7 @@ impl<'a> TableLayout<'a> {
style: &cell.base.style,
};
let mut positioning_context = PositioningContext::new_for_subtree(
collect_for_nearest_positioned_ancestor,
);
let mut positioning_context = PositioningContext::default();
let layout = cell.contents.layout(
layout_context,
&mut positioning_context,
@ -1503,7 +1475,6 @@ impl<'a> TableLayout<'a> {
layout_context: &LayoutContext,
parent_positioning_context: &mut PositioningContext,
) -> BoxFragment {
let mut positioning_context = caption.context.new_positioning_context();
let containing_block = &ContainingBlock {
size: ContainingBlockSize {
inline: self.table_width + self.pbm.padding_border_sums.inline,
@ -1517,6 +1488,8 @@ impl<'a> TableLayout<'a> {
// stretch block size. https://drafts.csswg.org/css-sizing-4/#stretch-fit-sizing
let ignore_block_margins_for_stretch = LogicalSides1D::new(false, false);
let mut positioning_context =
PositioningContext::new_for_layout_box_base(&caption.context.base);
let mut box_fragment = caption.context.layout_in_flow_block_level(
layout_context,
positioning_context
@ -1769,11 +1742,7 @@ impl<'a> TableLayout<'a> {
) -> BoxFragment {
self.distributed_column_widths =
Self::distribute_width_to_columns(self.assignable_width, &self.columns);
self.layout_cells_in_row(
layout_context,
containing_block_for_children,
positioning_context,
);
self.layout_cells_in_row(layout_context, containing_block_for_children);
let table_writing_mode = containing_block_for_children.style.writing_mode;
let first_layout_row_heights = self.do_first_row_layout(table_writing_mode);
self.compute_table_height_and_final_row_heights(
@ -2325,7 +2294,7 @@ impl<'a> RowFragmentLayout<'a> {
Self {
row: table_row,
rect,
positioning_context: table_row.base.new_positioning_context(),
positioning_context: PositioningContext::new_for_layout_box_base(&table_row.base),
containing_block,
fragments: Vec::new(),
}
@ -2379,11 +2348,11 @@ impl<'a> RowFragmentLayout<'a> {
if let Some(mut row_positioning_context) = self.positioning_context.take() {
row_positioning_context.layout_collected_children(layout_context, &mut row_fragment);
let positioning_context = row_group_fragment_layout
let parent_positioning_context = row_group_fragment_layout
.as_mut()
.and_then(|layout| layout.positioning_context.as_mut())
.unwrap_or(table_positioning_context);
positioning_context.append(row_positioning_context);
parent_positioning_context.append(row_positioning_context);
}
let fragment = Fragment::Box(ArcRefCell::new(row_fragment));
@ -2410,7 +2379,7 @@ impl RowGroupFragmentLayout {
let row_group = row_group.borrow();
(
dimensions.get_row_group_rect(&row_group),
row_group.base.new_positioning_context(),
PositioningContext::new_for_layout_box_base(&row_group.base),
)
};
Self {