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layout: Do not include position:fixed children when calculating scrollable overflow for root element (#38618)

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Reimplementation of: #35931

For a `FragmentTree` we define a scrollable overflow calculation that
includes the overflow all of it's children `Fragments`. In practice we
are using this calculation for scrolling area of the viewport and
defining the root scroll frames. However, since uncontained fixed
positioned element is located outside of the document and should not be
scrolled, and therefore it would make no sense to include them in the
calculation of its scrollable overflow as well.

Testing: New and existing WPT tests
Fixes: #38617
Fixes: #38182

---------

Signed-off-by: Jo Steven Novaryo <jo.steven.novaryo@huawei.com>
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Jo Steven Novaryo 2025-08-18 19:25:31 +08:00 • committed by GitHub
parent 7dcd89a6f9
commit 7489a0349f
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4 changed files with 58 additions and 5 deletions
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24
components/layout/fragment_tree/fragment_tree.rs
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@ -10,6 +10,7 @@ use compositing_traits::display_list::AxesScrollSensitivity;
use fxhash::FxHashSet;
use malloc_size_of_derive::MallocSizeOf;
use style::animation::AnimationSetKey;
use style::computed_values::position::T as Position;
use super::{BoxFragment, ContainingBlockManager, Fragment};
use crate::ArcRefCell;
@ -124,9 +125,26 @@ impl FragmentTree {
let scrollable_overflow = self.root_fragments.iter().fold(
self.initial_containing_block,
|overflow, fragment| {
fragment
.calculate_scrollable_overflow_for_parent()
.union(&overflow)
// Need to calculate the overflow for each fragments within the tree
// because it is required in the next stages of reflow.
let overflow_from_fragment =
fragment.calculate_scrollable_overflow_for_parent();
// Scrollable overflow should be accumulated in the block that
// establishes the containing block for the element. Thus, fixed
// positioned fragments whose containing block is the initial
// containing block should not be included in overflow calculation.
// See <https://www.w3.org/TR/css-overflow-3/#scrollable>.
if fragment
.retrieve_box_fragment()
.is_some_and(|box_fragment| {
box_fragment.borrow().style.get_box().position == Position::Fixed
})
{
return overflow;
}
overflow.union(&overflow_from_fragment)
},
);