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Code Issues Projects Releases Packages Wiki Activity Actions 10

LibWeb: Avoid computing automatic minimum size for some flex items

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There's a specific (and thankfully very common!) scenario where we can
actually skip calculating the automatic minimum size for flex items.

In single-line (no wrapping) flex containers, if the sum of all item
flex base sizes is <= the flex container's main size, we know that
none of the items will be shrunk by the layout algorithm.

And so for any flex item with definite main size AND automatic minimum
main size, we can treat the automatic minimum size as 0.
This commit is contained in:
Andreas Kling 2025-04-21 18:27:47 +02:00 • committed by Andreas Kling
commit 7b2a427430
Notes: github-actions[bot] 2025-04-22 13:47:03 +00:00
4 changed files with 366 additions and 10 deletions
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44
Libraries/LibWeb/Layout/FlexFormattingContext.cpp
View file

@ -1,5 +1,5 @@
/*
* Copyright (c) 2021-2024, Andreas Kling <andreas@ladybird.org>
* Copyright (c) 2021-2025, Andreas Kling <andreas@ladybird.org>
* Copyright (c) 2021, Tobias Christiansen <tobyase@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
@ -94,7 +94,38 @@ void FlexFormattingContext::run(AvailableSpace const& available_space)
// FIXME: Get rid of prepare_for_replaced_layout() and make replaced elements figure out their intrinsic size lazily.
static_cast<ReplacedBox&>(*item.box).prepare_for_replaced_layout();
}
determine_flex_base_size_and_hypothetical_main_size(item);
determine_flex_base_size(item);
}
// OPTIMIZATION: We try to avoid calculating the "automatic minimum size" if possible,
// since it may require expensive intrinsic sizing layout.
// If this is a single-line flex container and the sum of flex bases sizes
// won't exceed the available space in the main axis, we know the layout
// algorithm won't have to shrink anything, thus not needing the minimum size.
bool const should_skip_automatic_minimum_size_clamp = [&] {
if (m_layout_mode == LayoutMode::IntrinsicSizing)
return false;
if (!is_single_line())
return false;
if (is_row_layout() && !m_flex_container_state.has_definite_width())
return false;
if (!is_row_layout() && !m_flex_container_state.has_definite_height())
return false;
CSSPixels sum_of_item_flex_base_sizes = 0;
for (auto& item : m_flex_items) {
if (!has_definite_main_size(item))
return false;
sum_of_item_flex_base_sizes += item.flex_base_size;
}
if (sum_of_item_flex_base_sizes > m_available_space_for_items->main.to_px_or_zero())
return false;
return true;
}();
for (auto& item : m_flex_items) {
// The hypothetical main size is the item’s flex base size clamped according to its used min and max main sizes (and flooring the content box size at zero).
auto clamp_max = has_main_max_size(item.box) ? specified_main_max_size(item.box) : CSSPixels::max();
auto clamp_min = has_main_min_size(item.box) ? specified_main_min_size(item.box) : (should_skip_automatic_minimum_size_clamp ? 0 : automatic_minimum_size(item));
item.hypothetical_main_size = max(CSSPixels(0), css_clamp(item.flex_base_size, clamp_min, clamp_max));
}
if (available_space.width.is_intrinsic_sizing_constraint() || available_space.height.is_intrinsic_sizing_constraint()) {
@ -556,8 +587,8 @@ CSSPixels FlexFormattingContext::adjust_cross_size_through_aspect_ratio_for_main
return cross_size;
}
// https://www.w3.org/TR/css-flexbox-1/#algo-main-item
void FlexFormattingContext::determine_flex_base_size_and_hypothetical_main_size(FlexItem& item)
// https://drafts.csswg.org/css-flexbox-1/#algo-main-item
void FlexFormattingContext::determine_flex_base_size(FlexItem& item)
{
auto& child_box = item.box;
@ -694,10 +725,7 @@ void FlexFormattingContext::determine_flex_base_size_and_hypothetical_main_size(
item.flex_base_size = adjust_main_size_through_aspect_ratio_for_cross_size_min_max_constraints(child_box, item.flex_base_size, computed_cross_min_size(child_box), computed_cross_max_size(child_box));
}
// The hypothetical main size is the item’s flex base size clamped according to its used min and max main sizes (and flooring the content box size at zero).
auto clamp_min = has_main_min_size(child_box) ? specified_main_min_size(child_box) : automatic_minimum_size(item);
auto clamp_max = has_main_max_size(child_box) ? specified_main_max_size(child_box) : CSSPixels::max();
item.hypothetical_main_size = max(CSSPixels(0), css_clamp(item.flex_base_size, clamp_min, clamp_max));
// NOTE: We don't clamp the main size until we have them for all items.
}
// https://drafts.csswg.org/css-flexbox-1/#min-size-auto

4
Libraries/LibWeb/Layout/FlexFormattingContext.h
View file

@ -1,5 +1,5 @@
/*
* Copyright (c) 2021-2024, Andreas Kling <andreas@ladybird.org>
* Copyright (c) 2021-2025, Andreas Kling <andreas@ladybird.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
@ -173,7 +173,7 @@ private:
void determine_available_space_for_items(AvailableSpace const&);
void determine_flex_base_size_and_hypothetical_main_size(FlexItem&);
void determine_flex_base_size(FlexItem&);
void collect_flex_items_into_flex_lines();