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LibWeb: Do not require box to be positioned to create stacking context

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Instead of implementing stacking context painting order exactly as it
is defined in CSS2.2 "Appendix E. Elaborate description of Stacking
Contexts" we need to account for changes in the latest standards where
a box can establish a stacking context without being positioned, for
example, by having an opacity different from 1.

Fixes https://github.com/SerenityOS/serenity/issues/21137
This commit is contained in:
Aliaksandr Kalenik 2024-01-02 20:05:35 +01:00 committed by Alexander Kalenik
parent aa769d95df
commit 49fcc5dcd8
Notes: sideshowbarker 2024-07-17 23:07:41 +09:00 
Author: https://github.com/kalenikaliaksandr
Commit: https://github.com/SerenityOS/serenity/commit/49fcc5dcd8
Pull-request: https://github.com/SerenityOS/serenity/pull/22543
Issue: https://github.com/SerenityOS/serenity/issues/21137
Reviewed-by: https://github.com/awesomekling ✅
3 changed files with 55 additions and 25 deletions
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19
Tests/LibWeb/Ref/abspos-z-index-painting-order.html Normal file
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@ -0,0 +1,19 @@
<!doctype html><link rel="match" href="reference/abspos-z-index-painting-order-ref.html" /><style>
* {
font-size: 60px;
}
body {
/* NOTE: This is flex in order to force div.text to create a stacking context. */
display: flex;
}
.abspos {
position: absolute;
width: 100px;
height: 100px;
background: gray;
z-index: 0;
}
.text {
z-index: 1;
}
</style><body><div class="abspos"></div><div class="text">hello friends</div>

18
Tests/LibWeb/Ref/reference/abspos-z-index-painting-order-ref.html Normal file
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@ -0,0 +1,18 @@
<!doctype html><html lang="en"><style>
* {
font-size: 60px;
}
body {
display: grid;
grid-template-areas: "content";
}
.abspos {
grid-area: content;
background: gray;
width: 100px;
height: 100px;
}
.text {
grid-area: content;
}
</style><body><div class="abspos"></div><div class="text">hello friends</div></body>

43
Userland/Libraries/LibWeb/Painting/StackingContext.cpp
View file

@ -96,32 +96,14 @@ void StackingContext::paint_descendants(PaintContext& context, Paintable const&
paintable.for_each_child([&context, phase](auto& child) {
auto* stacking_context = child.stacking_context_rooted_here();
if (child.is_positioned()) {
// If `child` is positioned with a z-index of `0` or `auto`, skip over it.
auto const& z_index = child.computed_values().z_index();
if (!z_index.has_value() || z_index.value() == 0)
return;
// Skip positioned children with stacking contexts, these are handled in paint_internal().
if (stacking_context)
return;
}
if (stacking_context) {
// FIXME: This may not be fully correct with respect to the paint phases.
if (phase == StackingContextPaintPhase::Foreground)
paint_child(context, *stacking_context);
// Note: Don't further recuse into descendants as paint_child() will do that.
return;
}
auto const& z_index = child.computed_values().z_index();
// NOTE: Grid specification https://www.w3.org/TR/css-grid-2/#z-order says that grid items should be treated
// the same way as CSS2 defines for inline-blocks:
// "For each one of these, treat the element as if it created a new stacking context, but any positioned
// descendants and descendants which actually create a new stacking context should be considered part of
// the parent stacking context, not this new one."
auto should_be_treated_as_stacking_context = child.layout_node().is_grid_item();
auto should_be_treated_as_stacking_context = child.layout_node().is_grid_item() && !z_index.has_value();
if (should_be_treated_as_stacking_context) {
// FIXME: This may not be fully correct with respect to the paint phases.
if (phase == StackingContextPaintPhase::Foreground)
@ -129,6 +111,20 @@ void StackingContext::paint_descendants(PaintContext& context, Paintable const&
return;
}
if (stacking_context && z_index.has_value())
return;
if (child.is_positioned() && !z_index.has_value())
return;
if (stacking_context) {
// FIXME: This may not be fully correct with respect to the paint phases.
if (phase == StackingContextPaintPhase::Foreground) {
paint_child(context, *stacking_context);
}
// Note: Don't further recurse into descendants as paint_child() will do that.
return;
}
bool child_is_inline_or_replaced = child.is_inline() || is<Layout::ReplacedBox>(child);
switch (phase) {
case StackingContextPaintPhase::BackgroundAndBorders:
@ -207,9 +203,8 @@ void StackingContext::paint_internal(PaintContext& context) const
// Stacking contexts formed by positioned descendants with negative z-indices (excluding 0) in z-index order
// (most negative first) then tree order. (step 3)
// NOTE: This doesn't check if a descendant is positioned as modern CSS allows for alternative methods to establish stacking contexts.
for (auto* child : m_children) {
if (!child->paintable_box().is_positioned())
continue;
if (child->paintable_box().computed_values().z_index().has_value() && child->paintable_box().computed_values().z_index().value() < 0)
paint_child(context, *child);
}
@ -270,10 +265,8 @@ void StackingContext::paint_internal(PaintContext& context) const
// Stacking contexts formed by positioned descendants with z-indices greater than or equal to 1 in z-index order
// (smallest first) then tree order. (Step 9)
// NOTE: This doesn't check if a descendant is positioned as modern CSS allows for alternative methods to establish stacking contexts.
for (auto* child : m_children) {
if (!child->paintable_box().is_positioned())
continue;
PaintableBox const* nearest_scrollable_ancestor = child->paintable_box().nearest_scrollable_ancestor_within_stacking_context();
if (nearest_scrollable_ancestor)