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LibWeb: Improve graphical list item marker positioning

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While 788d5368a7 took care of better text
marker positioning, this improves graphical marker positioning instead.

By looking at how Firefox and Chrome render markers, it's clear that
there are three parts to positioning a graphical marker:

  * The containing space that the marker resides in;
  * The marker dimensions;
  * The distance between the marker and the start of the list item.

The space that the marker can be contained in, is the area to the left
of the list item with a height of the marker's line-height. The marker
dimensions are relative to the marker's font's pixel size: most of them
are a square at 35% of the font size, but the disclosure markers are
sized at 50% instead. Finally, the marker distance is always gauged at
50% of the font size.

So for example, a list item with `list-style-type: disc` and `font-size:
20px`, has 10px between its start and the right side of the marker, and
the marker's dimensions are 7x7.

The percentages I've chosen closely resemble how Firefox lays out its
list item markers.
This commit is contained in:
Jelle Raaijmakers 2025-07-16 21:35:09 +02:00 • committed by Sam Atkins
parent ada198bee0
commit 115e5f42af
Notes: github-actions[bot] 2025-07-17 08:36:21 +00:00
21 changed files with 187 additions and 171 deletions
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53
Libraries/LibWeb/Layout/BlockFormattingContext.cpp
View file

@ -831,9 +831,8 @@ void BlockFormattingContext::layout_block_level_box(Box const& box, BlockContain
compute_inset(box, content_box_rect(block_container_state).size());
// Now that our children are formatted we place the ListItemBox with the left space we remembered.
if (is<ListItemBox>(box)) {
if (is<ListItemBox>(box))
layout_list_item_marker(static_cast<ListItemBox const&>(box), left_space_before_children_formatted);
}
bottom_of_lowest_margin_box = max(bottom_of_lowest_margin_box, box_state.offset.y() + box_state.content_height() + box_state.margin_box_bottom());
@ -1192,24 +1191,26 @@ void BlockFormattingContext::ensure_sizes_correct_for_left_offset_calculation(Li
auto& marker = *list_item_box.marker();
auto& marker_state = m_state.get_mutable(marker);
CSSPixels image_width = 0;
CSSPixels image_height = 0;
// If an image is used, the marker's dimensions are the same as the image.
if (auto const* list_style_image = marker.list_style_image()) {
image_width = list_style_image->natural_width().value_or(0);
image_height = list_style_image->natural_height().value_or(0);
marker_state.set_content_width(list_style_image->natural_width().value_or(0));
marker_state.set_content_height(list_style_image->natural_height().value_or(0));
return;
}
CSSPixels marker_size = marker.relative_size();
marker_state.set_content_height(marker_size);
// Text markers use text metrics to determine their width; other markers use square dimensions.
auto const& marker_font = marker.first_available_font();
auto marker_text = marker.text();
if (!marker_text.has_value()) {
auto default_marker_width = max(4, marker.first_available_font().pixel_size() - 4);
marker_state.set_content_width(image_width + default_marker_width);
} else {
if (marker_text.has_value()) {
// FIXME: Use per-code-point fonts to measure text.
auto text_width = marker.first_available_font().width(marker_text.value().code_points());
marker_state.set_content_width(image_width + CSSPixels::nearest_value_for(text_width));
auto text_width = marker_font.width(marker_text.value().code_points());
marker_state.set_content_width(CSSPixels::nearest_value_for(text_width));
} else {
marker_state.set_content_width(marker_size);
}
marker_state.set_content_height(max(image_height, CSSPixels { marker.first_available_font().pixel_size() }));
}
void BlockFormattingContext::layout_list_item_marker(ListItemBox const& list_item_box, CSSPixels const& left_space_before_list_item_elements_formatted)
@ -1222,20 +1223,26 @@ void BlockFormattingContext::layout_list_item_marker(ListItemBox const& list_ite
auto& list_item_state = m_state.get_mutable(list_item_box);
auto marker_text = marker.text();
auto default_marker_width = marker_text.has_value() ? 0 : max(4, marker.first_available_font().pixel_size() - 4);
auto final_marker_width = marker_state.content_width() + default_marker_width;
// Text markers fit snug against the list item; non-text position themselves at 50% of the font size.
CSSPixels marker_distance = 0;
if (!marker_text.has_value())
marker_distance = CSSPixels::nearest_value_for(.5f * marker.first_available_font().pixel_size());
auto marker_height = marker_state.content_height();
auto marker_width = marker_state.content_width();
if (marker.list_style_position() == CSS::ListStylePosition::Inside) {
list_item_state.set_content_x(list_item_state.offset.x() + final_marker_width);
list_item_state.set_content_width(list_item_state.content_width() - final_marker_width);
list_item_state.set_content_x(list_item_state.offset.x() + marker_width + marker_distance);
list_item_state.set_content_width(list_item_state.content_width() - marker_width);
}
auto offset_y = max(CSSPixels(0), (marker.computed_values().line_height() - marker_state.content_height()) / 2);
auto offset_x = round(left_space_before_list_item_elements_formatted - marker_distance - marker_width);
auto offset_y = round(max(CSSPixels(0), (marker.computed_values().line_height() - marker_height) / 2));
marker_state.set_content_offset({ offset_x, offset_y });
marker_state.set_content_offset({ left_space_before_list_item_elements_formatted - final_marker_width, round(offset_y) });
if (marker_state.content_height() > list_item_state.content_height())
list_item_state.set_content_height(marker_state.content_height());
if (marker_height > list_item_state.content_height())
list_item_state.set_content_height(marker_height);
}
BlockFormattingContext::SpaceUsedAndContainingMarginForFloats BlockFormattingContext::space_used_and_containing_margin_for_floats(CSSPixels y) const