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LibWeb: Rewrite FFC "resolve flexible lengths" algorithm from draft spec

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The draft CSS-FLEXBOX-1 spec had a more detailed description of this
algorithm, so let's use that as our basis for the implementation.

Test by Aliaksandr. :^)
This commit is contained in:
Andreas Kling 2023-03-08 21:18:35 +01:00
parent 1cd61723f0
commit 109ed27423
Notes: sideshowbarker 2024-07-17 04:10:16 +09:00 
Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/109ed27423
4 changed files with 268 additions and 149 deletions
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29
Tests/LibWeb/Layout/expected/flex-column-height-unconstrained.txt Normal file
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@ -0,0 +1,29 @@
Viewport <#document> at (0,0) content-size 800x600 children: not-inline
BlockContainer <html> at (0,0) content-size 800x324 children: not-inline
BlockContainer <body> at (8,8) content-size 784x308 children: not-inline
Box <div.my-container.column> at (9,9) content-size 782x306 flex-container(column) children: not-inline
BlockContainer <(anonymous)> at (9,9) content-size 0x0 children: inline
TextNode <#text>
BlockContainer <div.box> at (10,10) content-size 100x100 flex-item children: inline
line 0 width: 6.34375, height: 17.46875, bottom: 17.46875, baseline: 13.53125
frag 0 from TextNode start: 0, length: 1, rect: [10,10 6.34375x17.46875]
"1"
TextNode <#text>
BlockContainer <(anonymous)> at (9,9) content-size 0x0 children: inline
TextNode <#text>
BlockContainer <div.box> at (10,112) content-size 100x100 flex-item children: inline
line 0 width: 8.8125, height: 17.46875, bottom: 17.46875, baseline: 13.53125
frag 0 from TextNode start: 0, length: 1, rect: [10,112 8.8125x17.46875]
"2"
TextNode <#text>
BlockContainer <(anonymous)> at (9,9) content-size 0x0 children: inline
TextNode <#text>
BlockContainer <div.box> at (10,214) content-size 100x100 flex-item children: inline
line 0 width: 9.09375, height: 17.46875, bottom: 17.46875, baseline: 13.53125
frag 0 from TextNode start: 0, length: 1, rect: [10,214 9.09375x17.46875]
"3"
TextNode <#text>
BlockContainer <(anonymous)> at (9,9) content-size 0x0 children: inline
TextNode <#text>
BlockContainer <(anonymous)> at (8,316) content-size 784x0 children: inline
TextNode <#text>

25
Tests/LibWeb/Layout/input/flex-column-height-unconstrained.html Normal file
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@ -0,0 +1,25 @@
<style>
body {
font-family: 'SerenitySans';
}
.my-container {
display: flex;
border: 1px solid salmon;
}
.column {
flex-direction: column;
}
.box {
width: 100px;
height: 100px;
border: 1px solid black;
}
</style>
<div class="my-container column">
<div class="box">1</div>
<div class="box">2</div>
<div class="box">3</div>
</div>

345
Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp
View File

@ -1,5 +1,5 @@
/*
* Copyright (c) 2021-2022, Andreas Kling <kling@serenityos.org>
* Copyright (c) 2021-2023, Andreas Kling <kling@serenityos.org>
* Copyright (c) 2021, Tobias Christiansen <tobyase@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
@ -869,176 +869,203 @@ void FlexFormattingContext::collect_flex_items_into_flex_lines()
m_flex_lines.append(move(line));
}
// https://www.w3.org/TR/css-flexbox-1/#resolve-flexible-lengths
void FlexFormattingContext::resolve_flexible_lengths()
// https://drafts.csswg.org/css-flexbox-1/#resolve-flexible-lengths
void FlexFormattingContext::resolve_flexible_lengths_for_line(FlexLine& line)
{
// 1. Determine the used flex factor.
// Sum the outer hypothetical main sizes of all items on the line.
// If the sum is less than the flex container’s inner main size,
// use the flex grow factor for the rest of this algorithm; otherwise, use the flex shrink factor
enum FlexFactor {
FlexGrowFactor,
FlexShrinkFactor
};
FlexFactor used_flex_factor;
// 6.1. Determine used flex factor
for (auto& flex_line : m_flex_lines) {
size_t number_of_unfrozen_items_on_line = flex_line.items.size();
CSSPixels sum_of_hypothetical_main_sizes = 0;
for (auto& item : flex_line.items) {
sum_of_hypothetical_main_sizes += (item.hypothetical_main_size + item.margins.main_before + item.margins.main_after + item.borders.main_before + item.borders.main_after + item.padding.main_before + item.padding.main_after);
auto used_flex_factor = [&]() -> FlexFactor {
CSSPixels sum = 0;
for (auto const& item : line.items) {
sum += item.outer_hypothetical_main_size();
}
if (sum_of_hypothetical_main_sizes < m_available_space_for_items->main.to_px_or_zero())
used_flex_factor = FlexFactor::FlexGrowFactor;
else
used_flex_factor = FlexFactor::FlexShrinkFactor;
if (sum < inner_main_size(flex_container()))
return FlexFactor::FlexGrowFactor;
return FlexFactor::FlexShrinkFactor;
}();
for (auto& item : flex_line.items) {
if (used_flex_factor == FlexFactor::FlexGrowFactor)
item.flex_factor = item.box.computed_values().flex_grow();
else if (used_flex_factor == FlexFactor::FlexShrinkFactor)
item.flex_factor = item.box.computed_values().flex_shrink();
// 2. Each item in the flex line has a target main size, initially set to its flex base size.
// Each item is initially unfrozen and may become frozen.
for (auto& item : line.items) {
item.target_main_size = item.flex_base_size;
item.frozen = false;
}
// 3. Size inflexible items.
for (FlexItem& item : line.items) {
if (used_flex_factor == FlexFactor::FlexGrowFactor) {
item.flex_factor = item.box.computed_values().flex_grow();
} else if (used_flex_factor == FlexFactor::FlexShrinkFactor) {
item.flex_factor = item.box.computed_values().flex_shrink();
}
// 6.2. Size inflexible items
auto freeze_item_setting_target_main_size_to_hypothetical_main_size = [&number_of_unfrozen_items_on_line](FlexItem& item) {
item.target_main_size = item.hypothetical_main_size;
number_of_unfrozen_items_on_line--;
// Freeze, setting its target main size to its hypothetical main size…
// - any item that has a flex factor of zero
// - if using the flex grow factor: any item that has a flex base size greater than its hypothetical main size
// - if using the flex shrink factor: any item that has a flex base size smaller than its hypothetical main size
if (item.flex_factor.value() == 0
|| (used_flex_factor == FlexFactor::FlexGrowFactor && item.flex_base_size > item.hypothetical_main_size)
|| (used_flex_factor == FlexFactor::FlexShrinkFactor && item.flex_base_size < item.hypothetical_main_size)) {
item.frozen = true;
};
for (auto& item : flex_line.items) {
if (item.flex_factor.has_value() && item.flex_factor.value() == 0) {
freeze_item_setting_target_main_size_to_hypothetical_main_size(item);
} else if (used_flex_factor == FlexFactor::FlexGrowFactor) {
// FIXME: Spec doesn't include the == case, but we take a too basic approach to calculating the values used so this is appropriate
if (item.flex_base_size > item.hypothetical_main_size) {
freeze_item_setting_target_main_size_to_hypothetical_main_size(item);
item.target_main_size = item.hypothetical_main_size;
}
}
// 4. Calculate initial free space
// Sum the outer sizes of all items on the line, and subtract this from the flex container’s inner main size.
// For frozen items, use their outer target main size; for other items, use their outer flex base size.
auto calculate_remaining_free_space = [&]() -> CSSPixels {
CSSPixels sum = 0;
for (auto const& item : line.items) {
if (item.frozen)
sum += item.outer_target_main_size();
else
sum += item.outer_flex_base_size();
}
return inner_main_size(flex_container()) - sum;
};
auto const initial_free_space = calculate_remaining_free_space();
// 5. Loop
while (true) {
// a. Check for flexible items.
// If all the flex items on the line are frozen, free space has been distributed; exit this loop.
if (all_of(line.items, [](auto const& item) { return item.frozen; })) {
break;
}
// b. Calculate the remaining free space as for initial free space, above.
line.remaining_free_space = calculate_remaining_free_space();
// If the sum of the unfrozen flex items’ flex factors is less than one, multiply the initial free space by this sum.
if (auto sum_of_flex_factor_of_unfrozen_items = line.sum_of_flex_factor_of_unfrozen_items(); sum_of_flex_factor_of_unfrozen_items < 1) {
auto value = initial_free_space * sum_of_flex_factor_of_unfrozen_items;
// If the magnitude of this value is less than the magnitude of the remaining free space, use this as the remaining free space.
if (abs(value) < abs(line.remaining_free_space))
line.remaining_free_space = value;
}
// c. If the remaining free space is non-zero, distribute it proportional to the flex factors:
if (line.remaining_free_space != 0) {
// If using the flex grow factor
if (used_flex_factor == FlexFactor::FlexGrowFactor) {
// For every unfrozen item on the line,
// find the ratio of the item’s flex grow factor to the sum of the flex grow factors of all unfrozen items on the line.
auto sum_of_flex_factor_of_unfrozen_items = line.sum_of_flex_factor_of_unfrozen_items();
for (auto& item : line.items) {
if (item.frozen)
continue;
float ratio = item.flex_factor.value() / sum_of_flex_factor_of_unfrozen_items;
// Set the item’s target main size to its flex base size plus a fraction of the remaining free space proportional to the ratio.
item.target_main_size = item.flex_base_size + (line.remaining_free_space * ratio);
}
} else if (used_flex_factor == FlexFactor::FlexShrinkFactor) {
if (item.flex_base_size < item.hypothetical_main_size) {
freeze_item_setting_target_main_size_to_hypothetical_main_size(item);
}
// If using the flex shrink factor
else if (used_flex_factor == FlexFactor::FlexShrinkFactor) {
// For every unfrozen item on the line, multiply its flex shrink factor by its inner flex base size, and note this as its scaled flex shrink factor.
for (auto& item : line.items) {
item.scaled_flex_shrink_factor = item.flex_factor.value() * item.flex_base_size.value();
}
auto sum_of_scaled_flex_shrink_factors_of_all_unfrozen_items_on_line = line.sum_of_scaled_flex_shrink_factor_of_unfrozen_items();
for (auto& item : line.items) {
// Find the ratio of the item’s scaled flex shrink factor to the sum of the scaled flex shrink factors of all unfrozen items on the line.
float ratio = 1.0f;
if (sum_of_scaled_flex_shrink_factors_of_all_unfrozen_items_on_line != 0)
ratio = item.scaled_flex_shrink_factor / sum_of_scaled_flex_shrink_factors_of_all_unfrozen_items_on_line;
// Set the item’s target main size to its flex base size minus a fraction of the absolute value of the remaining free space proportional to the ratio.
// (Note this may result in a negative inner main size; it will be corrected in the next step.)
item.target_main_size = item.flex_base_size - (abs(line.remaining_free_space) * ratio);
}
}
}
// 6.3. Calculate initial free space
auto calculate_free_space = [&]() {
CSSPixels sum_of_items_on_line = 0;
for (auto& item : flex_line.items) {
if (item.frozen)
sum_of_items_on_line += item.target_main_size + item.margins.main_before + item.margins.main_after + item.borders.main_before + item.borders.main_after + item.padding.main_before + item.padding.main_after;
else
sum_of_items_on_line += item.flex_base_size + item.margins.main_before + item.margins.main_after + item.borders.main_before + item.borders.main_after + item.padding.main_before + item.padding.main_after;
}
return m_available_space_for_items->main.to_px_or_zero() - sum_of_items_on_line;
};
// d. Fix min/max violations.
CSSPixels total_violation = 0;
CSSPixels initial_free_space = calculate_free_space();
flex_line.remaining_free_space = initial_free_space;
// Clamp each non-frozen item’s target main size by its used min and max main sizes and floor its content-box size at zero.
for (auto& item : line.items) {
if (item.frozen)
continue;
auto used_min_main_size = has_main_min_size(item.box)
? specified_main_min_size(item.box)
: automatic_minimum_size(item);
// 6.4 Loop
auto for_each_unfrozen_item = [&flex_line](auto callback) {
for (auto& item : flex_line.items) {
auto used_max_main_size = has_main_max_size(item.box)
? specified_main_max_size(item.box)
: NumericLimits<float>::max();
auto original_target_main_size = item.target_main_size;
item.target_main_size = css_clamp(item.target_main_size, used_min_main_size, used_max_main_size);
item.target_main_size = max(item.target_main_size, CSSPixels(0));
// If the item’s target main size was made smaller by this, it’s a max violation.
if (item.target_main_size < original_target_main_size)
item.is_max_violation = true;
// If the item’s target main size was made larger by this, it’s a min violation.
if (item.target_main_size > original_target_main_size)
item.is_min_violation = true;
total_violation += item.target_main_size - original_target_main_size;
}
// e. Freeze over-flexed items.
// The total violation is the sum of the adjustments from the previous step ∑(clamped size - unclamped size).
// If the total violation is:
// Zero
// Freeze all items.
if (total_violation == 0) {
for (auto& item : line.items) {
if (!item.frozen)
callback(item);
}
};
while (number_of_unfrozen_items_on_line > 0) {
// b Calculate the remaining free space
flex_line.remaining_free_space = calculate_free_space();
float sum_of_unfrozen_flex_items_flex_factors = 0;
for_each_unfrozen_item([&](FlexItem const& item) {
sum_of_unfrozen_flex_items_flex_factors += item.flex_factor.value_or(1);
});
if (sum_of_unfrozen_flex_items_flex_factors < 1) {
auto intermediate_free_space = initial_free_space * sum_of_unfrozen_flex_items_flex_factors;
if (abs(intermediate_free_space) < abs(flex_line.remaining_free_space))
flex_line.remaining_free_space = intermediate_free_space;
}
// c Distribute free space proportional to the flex factors
if (flex_line.remaining_free_space != 0) {
if (used_flex_factor == FlexFactor::FlexGrowFactor) {
float sum_of_flex_grow_factor_of_unfrozen_items = sum_of_unfrozen_flex_items_flex_factors;
for_each_unfrozen_item([&](FlexItem& item) {
float ratio = item.flex_factor.value_or(1) / sum_of_flex_grow_factor_of_unfrozen_items;
item.target_main_size = item.flex_base_size + (flex_line.remaining_free_space * ratio);
});
} else if (used_flex_factor == FlexFactor::FlexShrinkFactor) {
float sum_of_scaled_flex_shrink_factor_of_unfrozen_items = 0;
for_each_unfrozen_item([&](FlexItem& item) {
item.scaled_flex_shrink_factor = item.flex_factor.value_or(1) * item.flex_base_size.value();
sum_of_scaled_flex_shrink_factor_of_unfrozen_items += item.scaled_flex_shrink_factor;
});
for_each_unfrozen_item([&](FlexItem& item) {
float ratio = 1.0f;
if (sum_of_scaled_flex_shrink_factor_of_unfrozen_items != 0.0f)
ratio = item.scaled_flex_shrink_factor / sum_of_scaled_flex_shrink_factor_of_unfrozen_items;
item.target_main_size = item.flex_base_size - (abs(flex_line.remaining_free_space) * ratio);
});
}
} else {
// This isn't spec but makes sense.
for_each_unfrozen_item([&](FlexItem& item) {
item.target_main_size = item.flex_base_size;
});
}
// d Fix min/max violations.
CSSPixels adjustments = 0.0f;
for_each_unfrozen_item([&](FlexItem& item) {
auto min_main = has_main_min_size(item.box)
? specified_main_min_size(item.box)
: automatic_minimum_size(item);
auto max_main = has_main_max_size(item.box)
? specified_main_max_size(item.box)
: NumericLimits<float>::max();
CSSPixels original_target_size = item.target_main_size;
if (item.target_main_size < min_main) {
item.target_main_size = min_main;
item.is_min_violation = true;
}
if (item.target_main_size > max_main) {
item.target_main_size = max_main;
item.is_max_violation = true;
}
CSSPixels delta = item.target_main_size - original_target_size;
adjustments += delta;
});
// e Freeze over-flexed items
CSSPixels total_violation = adjustments;
if (total_violation == 0) {
for_each_unfrozen_item([&](FlexItem& item) {
--number_of_unfrozen_items_on_line;
item.frozen = true;
});
} else if (total_violation > 0) {
for_each_unfrozen_item([&](FlexItem& item) {
if (item.is_min_violation) {
--number_of_unfrozen_items_on_line;
item.frozen = true;
}
});
} else if (total_violation < 0) {
for_each_unfrozen_item([&](FlexItem& item) {
if (item.is_max_violation) {
--number_of_unfrozen_items_on_line;
item.frozen = true;
}
});
}
}
// 6.5.
for (auto& item : flex_line.items) {
item.main_size = item.target_main_size;
set_main_size(item.box, item.main_size.value());
// Positive
// Freeze all the items with min violations.
else if (total_violation > 0) {
for (auto& item : line.items) {
if (!item.frozen && item.is_min_violation)
item.frozen = true;
}
}
// Negative
// Freeze all the items with max violations.
else {
for (auto& item : line.items) {
if (!item.frozen && item.is_max_violation)
item.frozen = true;
}
}
// NOTE: This freezes at least one item, ensuring that the loop makes progress and eventually terminates.
flex_line.remaining_free_space = calculate_free_space();
// f. Return to the start of this loop.
}
// NOTE: Calculate the remaining free space once again here, since it's needed later when aligning items.
line.remaining_free_space = calculate_remaining_free_space();
// 6. Set each item’s used main size to its target main size.
for (auto& item : line.items) {
item.main_size = item.target_main_size;
set_main_size(item.box, item.target_main_size);
}
}
// https://drafts.csswg.org/css-flexbox-1/#resolve-flexible-lengths
void FlexFormattingContext::resolve_flexible_lengths()
{
for (auto& line : m_flex_lines) {
resolve_flexible_lengths_for_line(line);
}
}
@ -2089,4 +2116,24 @@ CSSPixelPoint FlexFormattingContext::calculate_static_position(Box const& box) c
return static_position_offset + diff;
}
float FlexFormattingContext::FlexLine::sum_of_flex_factor_of_unfrozen_items() const
{
float sum = 0;
for (auto const& item : items) {
if (!item.frozen)
sum += item.flex_factor.value();
}
return sum;
}
float FlexFormattingContext::FlexLine::sum_of_scaled_flex_shrink_factor_of_unfrozen_items() const
{
float sum = 0;
for (auto const& item : items) {
if (!item.frozen)
sum += item.scaled_flex_shrink_factor;
}
return sum;
}
}

18
Userland/Libraries/LibWeb/Layout/FlexFormattingContext.h
View File

@ -61,6 +61,21 @@ private:
float scaled_flex_shrink_factor { 0 };
float desired_flex_fraction { 0 };
CSSPixels outer_hypothetical_main_size() const
{
return hypothetical_main_size + margins.main_before + margins.main_after + borders.main_before + borders.main_after + padding.main_before + padding.main_after;
}
CSSPixels outer_target_main_size() const
{
return target_main_size + margins.main_before + margins.main_after + borders.main_before + borders.main_after + padding.main_before + padding.main_after;
}
CSSPixels outer_flex_base_size() const
{
return flex_base_size + margins.main_before + margins.main_after + borders.main_before + borders.main_after + padding.main_before + padding.main_after;
}
// The used main size of this flex item. Empty until determined.
Optional<CSSPixels> main_size {};
@ -91,6 +106,9 @@ private:
CSSPixels cross_size { 0 };
CSSPixels remaining_free_space { 0 };
float chosen_flex_fraction { 0 };
float sum_of_flex_factor_of_unfrozen_items() const;
float sum_of_scaled_flex_shrink_factor_of_unfrozen_items() const;
};
bool has_definite_main_size(Box const&) const;