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LibWeb: Introduce structure that maintains collapsible margins in BFC

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Previously y position of boxes in block formatting context
was calculated by looking at y position of previous in-flow
sibling and adding collapsed margin of "collapse through"
boxes lying between box currently being laid out and it's
previous in-flow sibling.

Here introduced BlockMarginState structure that maintains
array of currently collapsible margins hence we no longer
need to look at previous sibling to calculate y position
of a box.
This commit is contained in:
Aliaksandr Kalenik 2022-12-25 17:13:21 +03:00 • committed by Andreas Kling
parent e3d5b67eaf
commit fe8304d5de
5 changed files with 93 additions and 89 deletions
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67
Userland/Libraries/LibWeb/Layout/FormattingContext.cpp
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@ -927,7 +927,7 @@ Gfx::FloatPoint FormattingContext::calculate_static_position(Box const& box) con
} else {
x = m_state.get(box).margin_box_left();
// We're among block siblings, Y can be calculated easily.
y = compute_box_y_position_with_respect_to_siblings(box);
y = m_state.get(box).margin_box_top();
}
auto offset_to_static_parent = content_box_rect_in_static_position_ancestor_coordinate_space(box, *box.containing_block(), m_state);
return offset_to_static_parent.location().translated(x, y);
@ -1364,71 +1364,6 @@ float FormattingContext::containing_block_height_for(Box const& box, LayoutState
VERIFY_NOT_REACHED();
}
static Box const* previous_block_level_sibling(Box const& box)
{
for (auto* sibling = box.previous_sibling_of_type<Box>(); sibling; sibling = sibling->previous_sibling_of_type<Box>()) {
if (sibling->display().is_block_outside())
return sibling;
}
return nullptr;
}
float FormattingContext::compute_box_y_position_with_respect_to_siblings(Box const& box) const
{
auto const& box_state = m_state.get(box);
float y = box_state.border_box_top();
Vector<float> collapsible_margins;
auto* relevant_sibling = previous_block_level_sibling(box);
while (relevant_sibling != nullptr) {
if (!relevant_sibling->is_absolutely_positioned() && !relevant_sibling->is_floating()) {
auto const& relevant_sibling_state = m_state.get(*relevant_sibling);
collapsible_margins.append(relevant_sibling_state.margin_bottom);
// NOTE: Empty (0-height) preceding siblings have their margins collapsed with *their* preceding sibling, etc.
if (relevant_sibling_state.border_box_height() > 0)
break;
collapsible_margins.append(relevant_sibling_state.margin_top);
}
relevant_sibling = previous_block_level_sibling(*relevant_sibling);
}
if (relevant_sibling) {
// Collapse top margin with the collapsed margin(s) of preceding siblings.
collapsible_margins.append(box_state.margin_top);
float smallest_margin = 0;
float largest_margin = 0;
size_t negative_margin_count = 0;
for (auto margin : collapsible_margins) {
if (margin < 0)
++negative_margin_count;
largest_margin = max(largest_margin, margin);
smallest_margin = min(smallest_margin, margin);
}
float collapsed_margin = 0;
if (negative_margin_count == collapsible_margins.size()) {
// When all margins are negative, the size of the collapsed margin is the smallest (most negative) margin.
collapsed_margin = smallest_margin;
} else if (negative_margin_count > 0) {
// When negative margins are involved, the size of the collapsed margin is the sum of the largest positive margin and the smallest (most negative) negative margin.
collapsed_margin = largest_margin + smallest_margin;
} else {
// Otherwise, collapse all the adjacent margins by using only the largest one.
collapsed_margin = largest_margin;
}
auto const& relevant_sibling_state = m_state.get(*relevant_sibling);
return y + relevant_sibling_state.offset.y()
+ relevant_sibling_state.content_height()
+ relevant_sibling_state.border_box_bottom()
+ collapsed_margin;
} else {
return y + box_state.margin_top;
}
}
// https://drafts.csswg.org/css-sizing-3/#stretch-fit-size
float FormattingContext::calculate_stretch_fit_width(Box const& box, AvailableSize const& available_width) const
{