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LibWeb: Prevent margin double-counting with "collapse through" boxes

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If there is a remaining margin-bottom in margin collapsing state
tracker after laying out all boxes in the current BFC, it must be
assigned to the last in-flow child since margin collapsing cannot
occur across a formatting context boundary.

The current issue where margin-bottom may be counted twice due to
"collapse through" margins in the last in-flow child box is addressed
with this fix by excluding such boxes during the search for a box to
assign the remaining margin.

Test case coming with this fix has a layout bug with incorrectly
computed line height.
This commit is contained in:
Aliaksandr Kalenik 2023-04-27 04:34:32 +03:00 • committed by Andreas Kling
parent 7fffa0176f
commit 9ee64b5694
3 changed files with 37 additions and 9 deletions
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20
Userland/Libraries/LibWeb/Layout/BlockFormattingContext.cpp
View file

@ -48,6 +48,15 @@ CSSPixels BlockFormattingContext::automatic_content_height() const
return compute_auto_height_for_block_formatting_context_root(root());
}
static bool margins_collapse_through(Box const& box, LayoutState& state)
{
// FIXME: A box's own margins collapse if the 'min-height' property is zero, and it has neither top or bottom borders
// nor top or bottom padding, and it has a 'height' of either 0 or 'auto', and it does not contain a line box, and
// all of its in-flow children's margins (if any) collapse.
// https://www.w3.org/TR/CSS22/box.html#collapsing-margins
return state.get(box).border_box_height() == 0;
}
void BlockFormattingContext::run(Box const&, LayoutMode layout_mode, AvailableSpace const& available_space)
{
if (is<Viewport>(root())) {
@ -65,6 +74,8 @@ void BlockFormattingContext::run(Box const&, LayoutMode layout_mode, AvailableSp
for (auto* child_box = root().last_child_of_type<Box>(); child_box; child_box = child_box->previous_sibling_of_type<Box>()) {
if (child_box->is_absolutely_positioned() || child_box->is_floating())
continue;
if (margins_collapse_through(*child_box, m_state))
continue;
m_state.get_mutable(*child_box).margin_bottom = m_margin_state.current_collapsed_margin().value();
break;
}
@ -379,15 +390,6 @@ void BlockFormattingContext::layout_inline_children(BlockContainer const& block_
block_container_state.set_content_height(context.automatic_content_height());
}
static bool margins_collapse_through(Box const& box, LayoutState& state)
{
// FIXME: A box's own margins collapse if the 'min-height' property is zero, and it has neither top or bottom borders
// nor top or bottom padding, and it has a 'height' of either 0 or 'auto', and it does not contain a line box, and
// all of its in-flow children's margins (if any) collapse.
// https://www.w3.org/TR/CSS22/box.html#collapsing-margins
return state.get(box).border_box_height() == 0;
}
CSSPixels BlockFormattingContext::compute_auto_height_for_block_level_element(Box const& box, AvailableSpace const& available_space)
{
if (creates_block_formatting_context(box)) {