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LibWeb: Avoid leaking infinite remaining_free_space in FFC calculation

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After switching to fixed-point arithmetic in CSSPixels, it no longer
supports representing infinite values, which was previously the case
for remaining_free_space in FFC. Using Optional that is not empty only
when value is finite to store remaining_free_space ensures that
infinity is avoided in layout calculations.
This commit is contained in:
Aliaksandr Kalenik 2023-07-25 22:18:08 +02:00 committed by Andreas Kling
parent de95a2fe33
commit 152ce88984
Notes: sideshowbarker 2024-07-17 10:16:43 +09:00 
Author: https://github.com/kalenikaliaksandr
Commit: https://github.com/SerenityOS/serenity/commit/152ce88984
Pull-request: https://github.com/SerenityOS/serenity/pull/20197
4 changed files with 36 additions and 17 deletions
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8
Tests/LibWeb/Layout/expected/flex/inf-available-space-with-auto-margins.txt Normal file
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@ -0,0 +1,8 @@
Viewport <#document> at (0,0) content-size 800x600 children: not-inline
BlockContainer <html> at (0,0) content-size 800x33.46875 [BFC] children: not-inline
Box <body> at (8,8) content-size 784x17.46875 flex-container(column) [FFC] children: not-inline
BlockContainer <main> at (8,8) content-size 784x17.46875 flex-item [BFC] children: inline
line 0 width: 153.984375, height: 17.46875, bottom: 17.46875, baseline: 13.53125
frag 0 from TextNode start: 0, length: 13, rect: [8,8 153.984375x17.46875]
"hmmMMMMmmmmmm"
TextNode <#text>

10
Tests/LibWeb/Layout/input/flex/inf-available-space-with-auto-margins.html Normal file
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@ -0,0 +1,10 @@
<!DOCTYPE html><html><head><style>
body {
display: flex;
flex-direction: column;
}
main {
margin-top: auto;
margin-bottom: auto;
}
</style></head><body><main>hmmMMMMmmmmmm

33
Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp
View File

@ -949,7 +949,11 @@ void FlexFormattingContext::resolve_flexible_lengths_for_line(FlexLine& line)
// 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 {
auto calculate_remaining_free_space = [&]() -> Optional<CSSPixels> {
// AD-HOC: If the container is sized under max-content constraints, then remaining_free_space won't have
// a value to avoid leaking an infinite value into layout calculations.
if (available_main_size.might_be_saturated())
return {};
CSSPixels sum = 0;
for (auto const& item : line.items) {
if (item.frozen)
@ -978,16 +982,16 @@ void FlexFormattingContext::resolve_flexible_lengths_for_line(FlexLine& line)
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 (auto sum_of_flex_factor_of_unfrozen_items = line.sum_of_flex_factor_of_unfrozen_items(); sum_of_flex_factor_of_unfrozen_items < 1 && initial_free_space.has_value()) {
auto value = initial_free_space.value() * 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))
if (abs(value) < abs(line.remaining_free_space.value()))
line.remaining_free_space = value;
}
// AD-HOC: We allow the remaining free space to be infinite, but we can't let infinity
// leak into the layout geometry, so we treat infinity as zero when used in arithmetic.
auto remaining_free_space_or_zero_if_infinite = !line.remaining_free_space.might_be_saturated() ? line.remaining_free_space : 0;
auto remaining_free_space_or_zero_if_infinite = line.remaining_free_space.has_value() ? line.remaining_free_space.value() : 0;
// c. If the remaining free space is non-zero, distribute it proportional to the flex factors:
if (line.remaining_free_space != 0) {
@ -1094,11 +1098,6 @@ void FlexFormattingContext::resolve_flexible_lengths_for_line(FlexLine& line)
// 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();
// AD-HOC: Due to the way we calculate the remaining free space, it can be infinite when sizing
// under a max-content constraint. In that case, we can simply set it to zero here.
if (line.remaining_free_space.might_be_saturated())
line.remaining_free_space = 0;
// 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;
@ -1278,8 +1277,8 @@ void FlexFormattingContext::distribute_any_remaining_free_space()
// CSS-FLEXBOX-2: Account for gap between flex items.
used_main_space += main_gap() * (flex_line.items.size() - 1);
if (flex_line.remaining_free_space > 0) {
CSSPixels size_per_auto_margin = flex_line.remaining_free_space / static_cast<double>(auto_margins);
if (flex_line.remaining_free_space.has_value() && flex_line.remaining_free_space.value() > 0 && auto_margins > 0) {
CSSPixels size_per_auto_margin = flex_line.remaining_free_space.value() / auto_margins;
for (auto& item : flex_line.items) {
if (item.margins.main_before_is_auto)
set_main_axis_first_margin(item, size_per_auto_margin);
@ -1331,11 +1330,12 @@ void FlexFormattingContext::distribute_any_remaining_free_space()
} else {
initial_offset = 0;
}
if (number_of_items > 1)
space_between_items = flex_line.remaining_free_space / (number_of_items - 1);
if (flex_line.remaining_free_space.has_value() && number_of_items > 1)
space_between_items = flex_line.remaining_free_space.value() / (number_of_items - 1);
break;
case CSS::JustifyContent::SpaceAround:
space_between_items = flex_line.remaining_free_space / number_of_items;
if (flex_line.remaining_free_space.has_value())
space_between_items = flex_line.remaining_free_space.value() / number_of_items;
if (is_direction_reverse()) {
initial_offset = inner_main_size(flex_container()) - space_between_items / 2.0;
} else {
@ -1343,7 +1343,8 @@ void FlexFormattingContext::distribute_any_remaining_free_space()
}
break;
case CSS::JustifyContent::SpaceEvenly:
space_between_items = flex_line.remaining_free_space / (number_of_items + 1);
if (flex_line.remaining_free_space.has_value())
space_between_items = flex_line.remaining_free_space.value() / (number_of_items + 1);
if (is_direction_reverse()) {
initial_offset = inner_main_size(flex_container()) - space_between_items;
} else {

2
Userland/Libraries/LibWeb/Layout/FlexFormattingContext.h
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@ -109,7 +109,7 @@ private:
struct FlexLine {
Vector<FlexItem&> items;
CSSPixels cross_size { 0 };
CSSPixels remaining_free_space { 0 };
Optional<CSSPixels> remaining_free_space;
double chosen_flex_fraction { 0 };
double sum_of_flex_factor_of_unfrozen_items() const;